Why is it "make"?

Solution 1:

It doesn't seem as simple as the comments above suggest. The translator used transforms instead of transform. That means using make instead of makes was also intentional.

I want to love you simply, in signs not expressed: clouds to the rain which make them evanesce

You have to take "clouds to the rain" as an antecedent for the relative pronoun, which. In other words and more details;

tinder to the flame (antecedent here) which transforms it (it means tinder) to ash

clouds to the rain (antecedent here) which make them (them means clouds) evanesce

Tinder is something that causes flame and gets burned by "flame" to ashes. Literally, it causes its own death.

Clouds are things that cause rain and get washed out by "rain" to disappear. Literally, they also cause thier own death.

It is "very" logical and grammatical to use "make" instead of "makes"

Does it make sense to you? Hope it helps.