In A Doll’s House, the relationship between Mr. Krogstad and Mrs. Linde contrasts the marriage between Nora and Torvald Helmer. Krogstad and Linde’s union is based on honesty and forgiveness; on the other hand, the Helmers’ marriage is sustained by dishonesty and illusion.

Both Nora and Krogstad committed the same crime—forgery—for similar reasons but experienced very different consequences. When Torvald’s health is failing due to overwork, Nora forges her father’s signature in order to obtain funds for her husband’s convalescence abroad. Nora essentially saves Torvald’s life while hiding her crime. No one ever discovers her guilt, allowing Nora to preserve their marriage and her status as an innocent kept woman. Torvald continues to view her simply as a doll; she dresses up for him and plays his pretty housewife. She perpetuates the illusion of their marriage as pure (i.e., lacking any skeletons in the closet) and the illusion of their roles (Torvald as the adult and Nora as the child).

On the other hand, Krogstad’s reputation is marred when he is caught for committing forgery in order to support his family. Society considers him corrupt; people look upon him as “morally diseased.” Yet the crime is not hidden from Linde, who states, “I think the sick are those who most need taking care of.” In fact, despite discovering Krogstad’s crime—which he committed after she left him to marry a wealthy man in order to support her family—Linde still wishes to reunite with him.

LINDE: Is it too late now?

KROGSTAD: Christine, are you saying this deliberately? Yes, I am sure you are. I see it in your face. Have you really the courage, then—?

LINDE: I want to be a mother to someone, and your children need a mother. We two need each other. Nils, I have faith in your real character—I can dare anything together with you.

Linde forgives Krogstad for his crime. Torvald, on the other hand, cannot handle the truth and forgive Nora for her crime, even though she committed it in order to help him. Her confession shatters his illusion of her as his darling treasure; he berates her:

What a horrible awakening! All these eight years—she who was my joy and pride—a hypocrite, a liar—worse, worse—a criminal!

Also, Krogstad and Linde’s relationship is based on equality and partnership. Linde proposes to Krogstad:

Nils, how would it be if we two shipwrecked people could join forces? ... Two on the same piece of wreckage would stand a better chance than each on their own.

Linde will not give up her future position at the bank in order to save Krogstad’s job and pride; nonetheless, she will not hold her economic dominance over him but care for him and share her life with him.

I could not endure life without work. All my life, as long as I can remember, I have worked, and it has been my greatest and only pleasure. But now I am quite alone in the world—my life is so dreadfully empty and I feel so forsaken. There is not the least pleasure in working for one’s self. Nils, give me someone and something to work for.

In turn, Krogstad now feels loved, secure, and confident enough to declare, "Now I shall find a way to clear myself in the eyes of the world."

In contrast, the Helmers' marriage is based on inequality and a dynamic of dominance and submission. Torvald treats Nora as a possession he can control; Nora plays along until she realizes his lack of respect for her. At the end of the play, she declares,

I saw you were not the man I had thought you were. …

I was so absolutely certain, you would come forward and take everything upon yourself, and say: I am the guilty one.

She is aware that he does not truly love her as a wife or equal. Nora sees that he values himself over her and would not defend her if her crime were exposed ... a crime which she committed in order to save him. She tells him,

As soon as your fear was over—and it was not fear for what threatened me, but for what might happen to you—when the whole thing was past, as far as you were concerned it was exactly as if nothing at all had happened. Exactly as before, I was your little skylark, your doll, which you would in future treat with doubly gentle care, because it was so brittle and fragile. Torvald—it was then it dawned upon me that for eight years I had been living here with a strange man

Unlike Krogstad and Linde's relationship, the Helmers' marriage is doomed due to lack of transparency and respect between both parties.

In A Doll's House, how does Krogstad and Mrs. Linde's relationship highlight the Helmers' marriage?

Let's begin with Nora and Torvald. Here is what we witness throughout the play, which is backed by the fact that their relationship is over by the end of the play.

Nora and Torvald are not equals in the relationship. They are not a couple working together, but a couple where the man overpowers the woman.

On top of the lack of equity, there is also a lack of fairness in treatment. Torvald uses Nora as his wife, as the mother of his children, as an entertainer, and as his personal "doll" of sorts. He is patronizing and, in many occasions, condescending. In turn, Nora puts Torvald on a pedestal and would never be disrespectful to him.

There is nothing honest about Torvald and Nora's marriage. She acts for him, hides situations from him, goes behind his back even to eat a macaroon, and basically has to live a parallel life that meets his needs—rather than her real life.

Did you notice how cruel Torvald turned toward Nora when he saw that his ego, his reputation was potentially put at risk due to Nora's secret? The words that Torvald said to Nora seemed to come from a place where they have been brewing. It seems like Torvald had those words reserved for when the situation merited it. The cruel words he used were in no way justified. It symbolizes how much control Torvald had over Nora. He could make her or break her on the spot.

In contrast, Krogstad and Kristine Linde show a drastically different side of how relationships can work.

There is equality in their relationship. Both parties admit their wrongdoings in life. Both parents admit to be "shipwrecks." Both parties admit to be vulnerable and in need of help. Both parties agree to complement each other and helping each other out. Krogstad and Kristine treat each other fairly, taking into consideration their strengths and weaknesses.

There are no secrets between Linde and Krogstad. If anything, they are entirely open about their pasts and their present, which is why they have a chance at a future together.

Further, there is zero cruelty in Kristine and Krogstad's relationship. The man does not try to overpower the woman with insults or hurtful statements. Linde is really straightforward with Krogstad, and vice versa.

It is ironic that the picture-perfect Helmers, who have every resource available to make each other happy, are the easiest to break apart, whereas the two "shipwrecks" who lack money, jobs, status, and class distinctions found one another and are ready to become a strong couple.

Krogstad and Mrs. Linde come together under peculiar circumstances in A Doll's House. Importantly, Mrs. Linde holds a position of power in relation to Krogstad when she invites him to meet at the Helmer house in Act III. She now holds the job which once belonged to Krogstad and she knows everything about his treatment of Nora, his loan to her and the blackmail he has attempted. 

Krogstad, on the other hand, is in a powerless position. Jobless and stripped of his honor and reputation, Krogstad has nothing to offer his children and no prospects for improving his situation. 

Mrs. Linde, after losing her husband, wants to be needed again and to have someone to take care of. Krogstad and his children fit her desires and she makes an offer to Krogstad. In their conversation Mrs. Linde makes her motives clear. 

Mrs. Linde's position of power over Krogstad and her clarity of purpose are both in contrast to Nora's relationship to Torvald. 

Keeping secrets, breaking his rules, and feeling diminished by Torvald, Nora only speaks her mind when it is too late. There is no clarity in her relations with Torvald. Subterfuge and manipulation are her main tools when dealing with her husband - a stark difference from the way Mrs. Linde openly deals with Krogstad. 

Nora resorts to manipulation because she has no real power in the her relationship with Torvald. 

She is viewed as an object, a toy, a child, but never an equal.

In "A Doll's House," how does Krogstad and Mrs. Linde's relationship highlight the Helmers' marriage qualities?

In "A Doll's House," there are certain conventions and expectations which are prevalent throughout. Nora would never be permitted to borrow money without her husband's consent and she would not be allowed to work unless she was either unmarried or a widow. Accordingly, Nora has had to take extraordinary steps to secure a loan, without Torvald's knowledge and to work and save what she can so as to be able to pay off the loan. It is almost paid off but now Krogstad (who arranged Nora's loan in the first place and knows her secret) has re-emerged and threatens Nora and Torvald's seemingly perfect life as he has threatened to expose her. At first, Nora is confident that her husband "will of course pay you off at once," if necessary but she would rather he did not find out about it this way. 

Nora and Torvald's marriage is based on his need to protect his "little spendthrift," whom he treats like a child, and her willingness to reduce herself to his "little squirrel," subject to his rules, and with no apparent contribution to make to family business. Torvald is condescending and is fixed in his belief of how a family should be run. Only when Krogstad and Mrs. Linden are reunited, is Nora forced to face the fact that her own marriage does not have a solid foundation. Torvald feels so strongly about what Krogstad may have done (which is ironically exactly what Nora has done) that he suggests that, "Every breath the children draw contains some germ of evil." Torvald says that Krogstad is "morally ruined" for failing to admit his wrongdoing and face the consequences. Nora is terrified at the thought that she may be corrupting her own children. This reveals the complete disparity between Nora and Torvald and foreshadows what will follow. 

Torvald says that "my shoulders are broad enough to bear the whole burden" because he does not think that anything Nora has done could ever be significant. His words are hollow and do not provide Nora with sufficient confidence to tell him everything. Their marriage has always been based on Nora's complete submission. On the other hand, even though Mrs. Linden has only just returned to the town, she is already confident that she can talk to Krogstad, and she tells Nora that she will convince him to demand his letter back. Nora begins to feel more confident, waiting for the "miracle."

Krogstad and Christina are honest with each other, and this emphasizes the lack of faith in the Helmer marriage. Furthermore, even though they have hurt each other in the past, if they are to make a new start, it will be on equal footing. This is in stark contrast to Torvald and Nora and highlights their insecurities. They cannot make a new start because they still cannot be wholly honest and Torvald cannot see his own failings. He cannot see Nora's worth. 

Krogstad and Christina may still have some reservations about their future but they prove their devotion to each other. Christina is prepared to withdraw her request to Krogstad to prove that she is genuine, and at the same time, Krogstad will withdraw the letter if it is what Christina wants. The decision to leave the letter, if the Helmers' are to have any chance of repairing their marriage, is based on Christina's view. Unfortunately, the Helmers' do not have the foundation to recover from. 

The relationship between Krogstad and Mrs. Linde illustrates how forgiveness can heal relationships and bring people together. Krogstad and Mrs. Linde had a relationship many years before, but Mrs. Linde married someone that would financially support her mother and brothers. She did what she had to do for her family's sake, and years later, Krogstad was able to forgive her and move past the pain to go forward in their relationship.

This is a direct contrast to Nora and Torvald. Nora lied and forged for the good of her family, but Torvald is unable to forgive her and Nora realizes that her marriage cannot be healed.