وَنَجِّنَا (And save us) is a command — it's telling someone to do something.

We translate it as a direct command — "[Do this]!"

وَنَجِّنَا (And save us) uses form II (فَعَّلَ) — the middle letter is doubled, which makes the meaning stronger or causative.

The meaning is more intense than the basic form, or means "to make someone do [action]".

وَنَجِّنَا (And save us) starts with وَ, the most common Arabic word — it means "and".

We add "and" to connect this to what came before.