Quiet!

Be quiet! = stop talking.

Can you keep quiet while I'm on the phone? = Don't make so much noise.

The town centre was very quiet today. = It wasn't busy.

You are very quiet today. What's the matter? = You aren't speaking much. What's wrong?

I'm looking forward to the peace and quiet. = I'm going to a place where there isn't much activity or excitement and no stress

Why did you keep quiet about your new job? = Why didn't you tell anybody about it?

 ¡estáte quieto! = Don't move! Stay still! Stand still! Sit still! Stop fidgeting! (NOT Be quiet!)

He's always very calm. (tranquilo) = He doesn't get angry or stressed out.

She's so laid-back. (tranquilo) = She is really relaxed about things. She doesn't let things worry her.

Tranquilo! = Relax! (NOT Quiet! and NOT Silence!)

Tú, tranquilo, lo haré yo = Don't worry! I'll do it.

Ahora que estás aqui estoy más tranquilo = I feel much better now that you are here. (NOT more quiet)

Someone wrote to me saying "I hope that this weekend I'll have time to read your blog quietly."

She should have said "calmly" or "without any rush".

To summarise, don't always use quiet to translate the idea of quieto or tranquilo.