Answer & Explanation:
The sonnet is written in the form of three quartets and a final couplet that closes up the sonnet. Each quartet is made of 4 lines and the final couplet has only two lines.
The rhyme scheme is a typical ABAB in each quartet, which means that the first line's end rhymes with the third and the second line with the fourth, in every quartet, and finally, the couplet rhymes the two lines' ends.
The main idea of the poem is that men should not give up on a woman's love even if she doesn't initially correspond it, and also, that the harder the better in the matter of love.
The author refers to the "burning an oak" to express how having the loved one's attention is progressive, as the oak, the fire does not immediately burn it all, but it starts to consume it bit by bit, therefore, having the woman's love is similar, and requires effort and patience. Also, as the process is similar to a burning oak, it starts from outside and goes deeper and deeper as the fire grows, indicating how love also grows from outside to inside.