Learn Icelandic: Stoppið núna!

The -ið ending tells us this is a plural command. You are telling two or more people to stop. If you were speaking to just one person, you would typically use the singular command, stoppaðu.

Do I need to add the word for 'you' (plural) to this command?

No, you do not need to add þið (you, plural). The -ið ending already makes it clear that you are talking to multiple people. While singular commands often attach the pronoun to the end of the verb, plural commands usually stand perfectly well on their own.

Is stoppið based on an English loanword? It looks so familiar!

Yes, the infinitive verb is að stoppa. Icelandic sometimes borrows words from other languages. Even though it is borrowed, it still follows regular Icelandic grammar rules, which is why it takes the standard plural command ending -ið here.

Why use núna instead of nú for 'now'?

Both mean 'now', but núna emphasizes this exact, immediate moment. That makes it the perfect choice for a sudden command. Nú is often used for broader timeframes, like 'nowadays', or just as a conversational filler.