Learn Croatian: Sjednem na stolicu i čitam. - I sit down on the chair and read.

Sjednem comes from the verb sjesti, which means to sit down (the physical action of taking a seat). On the other hand, sjedim comes from sjediti, which means to be sitting (the continuous state of already being in a seat). Since the speaker is performing the action of moving into the seat before reading, sjednem is the correct choice.

Why is the word for chair stolicu and not stolici?

The preposition na can take either the accusative or the locative case depending on whether there is movement. Because the action of sitting down involves motion towards a destination, we use the accusative case stolicu. If you were already resting on the chair with no motion involved, you would use the locative case stolici.

Why is the word for I (ja) missing from the beginning of the sentence?

In Croatian, subject pronouns like ja are usually dropped. The verbs sjednem and čitam both end in -m, which already makes it perfectly clear who is doing the action. You would only add ja if you really needed to emphasize that it is you doing it rather than someone else.

The sentence mixes a perfective verb (sjednem) and an imperfective verb (čitam). Is this normal?