What word means to decide calm or quiet?

There isn't a single word that perfectly captures the meaning of "to decide calm or quiet." However, depending on the context, you could use words like:

For "decide calm":

* Soothe: To make someone feel calmer or less upset.

* Pacify: To make someone calm and peaceful.

* Compose: To calm oneself or someone else down.

* Tranquilize: To make someone calm or still.

For "decide quiet":

* Hush: To make someone be quiet.

* Silence: To make something completely quiet.

* Muzzle: To prevent someone from speaking.

* Suppress: To stop something from being expressed or heard.

The best word to use will depend on the specific situation and what you are trying to convey.