Learn Polish: Kameleon idzie powoli, tak jakby udawał liść. - The chameleon walks slowly, as if it were pretending to be a leaf.

In Polish, clauses introduced by jakby or tak jakby (as if) require a past tense form to show that a situation is hypothetical or unreal. This is very similar to how English uses the past tense in phrases like as if it were.

Is there a difference between using jakby and tak jakby in this sentence?

No, the core meaning is exactly the same. Adding tak just gives a little more emphasis to the comparison and often makes the sentence rhythm flow better. You could easily drop it and just say jakby udawał liść.

Why is liść not in the Instrumental case here, since the translation says pretending to be a leaf?

The verb udawać (to pretend) takes a direct object in the Accusative case. In Polish, you don't pretend to be something; you just directly pretend that thing. Because liść is a masculine inanimate noun, its Accusative form is identical to its Nominative form.

Why use idzie instead of chodzi?

We use idzie because it is a determinate verb of motion, meaning the chameleon is walking right now, in a single ongoing direction. If we wanted to talk about its general habit of walking slowly on a regular basis, we would switch to the indeterminate verb chodzi.