it is once he betrayed his oath to Hermia by making one for her, Lysander's promises canceled out themselves.

Then, after declaring Lysander's oaths void and worth nothing, Helena says that both are "light as tales". The word "tales" might be an allusionto quick, fictional short stories that people make up to entertain others, or may also refer to fibs told in a moment of desperation or confusion. Shakespeare would have been aware of Chaucer's Canterbury Tales which are stories made up by a diverse group of people to entertain each other on their pilgrimage. Many of the tales are fabricated in order to glorify the storyteller. At any rate, Helena isn't buying what Lysander is selling.

One of Helena's important quotes is spoken as a soliloquy after she has overheard Hermia and Lysander agree to meet and run away together. Demetrius used to love Helena before he saw and fell in love with Hermia. Helena is determined to win him back. She knows she is as attractive as Hermia, but Demetrius doesn't think she's pretty now that he's fallen for Hermia. Helena says:

Things base and vile, holding no quantity,
Love can transpose to form and dignity:
Love looks not with the eyes, but with the mind;
And therefore is wing'd Cupid painted blind:
Nor hath Love's mind of any judgement taste;
Wings and no eyes figure unheedy haste:
And therefore is Love said to be a child,
Because in choice he is so oft beguiled.

This is a way of saying that beauty is in the eye of the beholder. If you love someone, you think they're beautiful, which means that love isn't looking with the eyes "but with the mind." Helena says the Greek god of love, Cupid, is depicted as being blind for that very reason. This isn't to say love is logical. Indeed, she says, it has no taste of judgment—that is, discernment. This is another way of saying that there is no accounting for taste, a remark often made about why two people end up together. Cupid is blind, but he has wings, meaning that love rushes in blindly—again suggesting that love is not rational.

Ironically, although Helena is able to discern that love for Hermia is making Demetrius behave like a fool, she has no insight into how her love for Demetrius degrades her own behavior. Consequently, she delivers a speech declaring her love for Demetrius that is quite cringe-worthy. When Demetrius tells her he can't love her, she replies:

And even for that do I love you the more.
I am your spaniel; and, Demetrius,
The more you beat me, I will fawn on you:
Use me but as your spaniel, spurn me, strike me,
Neglect me, lose me; only give me leave,
Unworthy as I am, to follow you.
What worser place can I beg in your love—
And yet a place of high respect with me—
Than to be used as you use your dog?

Obviously, it's an unhealthy kind of "love" that allows or even asks the loved one to disrespect and abuse you. Helena's two quotes leave modern viewers with mixed feelings about Helena. She seems to have some understanding of love's pitfalls but not when it comes to herself. This is a satirical look at young love, exposing the foolishness of the infatuation that often captures young people's emotions and stifles clear thinking.

Although the plot concerning the two pairs of young lovers in A Midsummer Night's Dream by William Shakespeare is highly entertaining and drives our introduction to the magical world of the forest, the young lovers themselves are not actually among the most interesting or fully developed of Shakespeare's characters. Motivated by her infatuation with Demetrius, she is willing to betray her childhood friend Hermia with no qualms at all, and acts mainly to advance her own personal ends, having little empathy or understanding for the other characters of the play. In the comic cat-fight between Hermia and Helena, we find out that one of her distinguishing characteristics is that she is tall and Hermia short. Her pathetic chasing after Demetrius despite his fickleness suggests a lack of self-respect. 

One quotation that illustrates Helena's character, emphasizing her tendency to self-pity, comes from Act II Scene 2:

Wherefore was I to this keen mockery born?

When at your hands did I deserve this scorn?...