"I think that this actress plays well."
Translation:אני חושבת שהשחקנית הזאת משחקת טוב.
What does היטב mean, how is that formed? Is it a process you can apply to other adjectives as well?
It's היטב heitev. It means "well", and no, it can't be applied to other words.
Because it's used as an adverb here, I guess. Adverbs usually don't change for gender or number, as far as I know. Can a native speaker confirm with more examples?