Learn Croatian: Sjedni na stolicu.

It addresses one person informally (like a friend or a child). It is the command form for ti (you, singular). If you were speaking to a group of people, or addressing someone formally, you would add -te to the end to make it Sjednite.

How is the command form sjedni created?

To make a command, we usually take the 'they' form of the verb in the present tense and drop the ending. The verb for 'to sit down' is sjesti, and its 'they' form is sjednu. We drop the -u and add -i to get the command sjedni.

Why is the word for chair stolicu instead of its dictionary form stolica?

We use the accusative case here, changing the feminine -a ending to -u, because the action involves motion towards a destination. You are moving your body onto the chair. If you were already resting there and not moving, we would use a different case to describe your static location.

I have seen the verb sjediti used for sitting. Why aren't we using its command form here?