Learn Spanish (Latin America): Nos sentamos cerca de la puerta.

In Spanish, the verb for 'to sit down' is sentarse, which is a reflexive verb. Literally, it means 'to seat oneself.' Because 'we' are doing the sitting, we use the reflexive pronoun nos (ourselves). If you just say sentamos without nos, it means you are seating someone else, like a child.

I learned that this verb has an 'ie' in it, like me siento (I sit). Why doesn't sentamos have an 'ie'?

You are right that sentarse is a stem-changing verb where the e changes to ie. However, in the present tense, the nosotros (we) form never gets this stem change. It stays regular, which is why it is nos sentamos and not nos sientamos.

Why is there a de after cerca? Doesn't cerca already mean 'near'?

Yes, cerca means 'near' or 'close.' But in Spanish, when you want to say what something is near to, you use the fixed phrase cerca de. You can think of it like the English phrase 'close to' — you wouldn't say 'we sit close the door,' you need the 'to' (de).

Is it wrong to put nosotros at the beginning of this sentence?