Learn Spanish (Latin America): No hables fuerte porque el bebé duerme. - Do not speak loudly because the baby is sleeping.

This is a negative command. When telling someone informally (using tú) not to do something, Spanish verbs swap their normal endings. For -ar verbs like hablar, the ending changes from -as to -es. So, "you speak" is hablas, but "don't speak" is no hables.

I thought fuerte meant "strong". Why does it mean "loudly" here?

Fuerte is usually the adjective for "strong", but it is very commonly used as an adverb to mean "loud" or "loudly" when talking about sounds, voices, or music. You could also use alto here (no hables alto).

The English translation says "is sleeping", so why use duerme instead of está durmiendo?

Spanish often uses the simple present tense (duerme) to describe actions happening right now, while English prefers the "-ing" form ("is sleeping"). You absolutely can say el bebé está durmiendo, but el bebé duerme is just as natural and very common.

Is the accent mark on bebé important?