What's the Big Idea?

Taking Peterson’s poem exercise I learned a few things about myself. I understand the main idea of poems but I have no skills handling the mechanics of poetry. I know that “gloaming” must be dusk because that comes before night, but assonance means nothing to me. This exercise left me with a few questions; How can a word’s vowel rhyme but the word itself does not? How do you know which lines in the sonnet are a couplet? Why are mechanics important in understanding poetry if I know the big picture?