What is the main conflict of The Crucible? Is it internal or external?
I'd argue that the main conflict of The Crucible is John Proctor's internal struggle with himself. He spends the entirety of the play trying not to feel like a "fraud." Though he does seem to have some remaining feelings for Abigail -- he does say that he "may think of [her] softly from time to time" -- he refuses to continue his affair with her because he will not be unfaithful to his wife again. His unwillingness to confess his guilt to the town prevents him from revealing what Abigail shared with him, that the girls "were dancin' in the woods [...], and [her] uncle leaped in on [them]. [Betty] took fright is all." In other words, witchcraft is not to blame for Betty Parris's illness.
However, by the time Proctor realizes that Abigail means to accuse his wife, she has already done it. Now it is too late for him to confess his relationship with her without looking like he's making it all up in order to save Elizabeth's life. He must live with the fact that, by keeping silent, he permitted his wife to be accused, convicted, and jailed. Only her pregnancy saves her from the noose. Now he has wronged her twice: once with the original act of infidelity and again when he refused to speak up until it was too late.
In the end, it is clear that John believes that he is a "no good man." He considers confessing to witchcraft, a lie, in order to live because "Nothing's spoiled by giving them this lie that were not rotten long before." John believes that he is already so soiled by his former sins that it will do no harm to commit one more. But, when it comes time to sign his confession, he finds that he is not willing to sacrifice his integrity. He rescinds his lies and says, "You have made your magic now, for now I do think I see some shred of goodness in John Proctor." He finally takes a stand, and so he redeems himself in his own eyes.