"There are still many green leaves in the tree."
Translation:Puussa on vielä monta vihreää lehteä.
It would have to be a different sentence then, vihreä lehti would have to be in plural partitive, not singular, which isn’t even covered in this course. Check out the use of monta vs paljon here https://www.finlandforum.org/viewtopic.php?t=64224
It actually is formed in different ways, there are too many rules to describe in a comment. Here is a brief article on that, I think it covers most of the basics https://uusikielemme.fi/finnish-grammar/finnish-cases/grammatical-cases/the-partitive-case-partitiivi
Kiitos. This is a very useful link. For my question, the answer is as follows:
Adjectives fairly regularly end in -ea/eä. In these cases, we can use both the ending -a/-ä and -ta/-tä. The single -a is much more common in current Finnish, so I would suggest using that form. In spoken language, you will hear vaikeeta, tärkeetä and nopeeta, with a long vowel sound.