Couplets can be divided into positive pairs, negative pairs and flow pairs, definite sentence pairs and Julian pairs. It can also be divided into work pairs and scattered pairs, but no matter how it changes, it can't escape the following principles: 1, the number of words is the same, and the parts of speech are similar (function words are function words and content words are content words). Such as: drizzle/fish/out, breeze/swallow/oblique. Noun/noun/verb, noun/noun/verb.
2, level and level, the rhythm is consistent. (That's what you said, flat and even, flat and even, just rhymes. Like writing poetry, in level tone, it is generally clear that 246 is no matter what 135 is. The rhythm is that the upper and lower couplets should be consistent. Such as: Wan Li/Sad Autumn/Frequently/Quietly, Centennial/Sick/Alone/On Stage (Quietly).
3, the content is related, and the artistic conception of the up and down links should not be too abrupt.