If you are sitting somewhere, for example in a chair, your bottom is resting on the chair and the upper part of your body is upright.

When you sit somewhere, you lower your body until you are sitting on something.

If you sit someone somewhere, you tell them to sit there or put them in a sitting position.

If you sit for an artist or photographer, you place yourself in a sitting position so you can be painted or photographed.

If you sit an examination, you do it.

If you sit on a committee or other official group, you are a member of it.

When a parliament, legislature, court, or other official body sits, it officially carries out its work.

If a building or object sits in a particular place, it is in that place.

To sit for someone means the same as to babysit for them.

If you sit tight, you remain in the same place or situation and do not take any action, usually because you are waiting for something to happen.