Learn Finnish: Kukaan ei pyyhi lehtiä, mutta kasvi kukoistaa silti. - Nobody wipes the leaves, but the plant flourishes anyway.

Because the first part of the sentence is negative (Kukaan ei pyyhi). In Finnish, the object of a negative sentence is always in the partitive case. Lehtiä is the partitive plural of lehti (leaf).

I know kukaan means "anyone", so why does the translation say "nobody"?

Finnish doesn't have a single standalone word for "nobody". Instead, you use the pronoun kukaan together with the negative verb ei. Literally, kukaan ei pyyhi means "anyone does not wipe", which is how Finnish expresses that "nobody wipes".

What is the basic form of pyyhi and why does it look different here?

The dictionary form is pyyhkiä (to wipe). Because it follows the negative verb ei, it must be in the connegative form. This form is built like the "minä" form but without the "-n" ending (minä pyyhin -> pyyhi). This triggers consonant gradation, softening the strong hk to just h.

Could I use kuitenkin instead of silti at the end of the sentence?

Yes, both words work perfectly here to mean "anyway", "still", or "nevertheless". Silti is very common in spoken Finnish because it is shorter and provides a punchy way to emphasize a contrast.