Learn German: Hinter der Mauer sind wir sicher.

hinter is a two-way preposition. When it describes a fixed location or state (answering 'where?' or wo?), it takes the dative case. If it were describing movement to a new destination (answering 'to where?' or wohin?), like running to hide behind the wall, it would take the accusative (die Mauer).

Why does the verb sind come before the subject wir?

This is a classic example of German's verb-second (V2) rule. The prepositional phrase Hinter der Mauer takes up the first position in the sentence. The conjugated verb sind must be in the second position, which naturally forces the subject wir into the third position.

How does the nuance change if I say Wir sind sicher hinter der Mauer instead?

The core meaning is exactly the same, but moving Hinter der Mauer to the front of the sentence adds emphasis to the location. It highlights that the safety specifically comes from being behind the wall, perhaps in contrast to being exposed out in the open.

I learned that sicher means 'sure' or 'certain'. How do we know it means 'safe' here?

sicher is a versatile word used for 'safe', 'secure', 'sure', and 'certain'. Context is your best guide here. If you actually wanted to express 'We are sure' about a fact, you would typically use a slightly different structure with a reflexive pronoun: Wir sind uns sicher.