What does the first sentence of a poem do?

The first sentence of ancient poetry is usually about time, place, environment, description, or narration, and some of them are lyrical from the beginning. If human nature is good, I am evil. But according to the connecting structure, most of them will show ambition until the end. There is also a poem that turns decay into magic, such as two or three or four paragraphs, which will not enter the poem until later. Generally speaking, there are not many words in the poem, and there should be no nonsense in the poem.