Learn Croatian: Uzmi jaknu. - Take the jacket.

This is the imperative (command) form used when speaking informally to one person. To make this form, you usually take the present tense base of the verb and add -i. The verb for 'to take' is uzeti, its present tense base is uzm-, so the command becomes uzmi.

How would I say 'Take the jacket' to a group of people, or formally to one person?

You would add -te to the singular command form. So, uzmi becomes uzmite. The full sentence would be Uzmite jaknu.

Why is the word for jacket spelled jaknu instead of jakna?

Because the jacket is the direct object of the action, meaning it is the thing being taken. In Croatian, feminine nouns that end in -a in their dictionary form, like jakna, change their ending to -u when they are the direct object. This is a refresher on the accusative case you learned earlier.

Do I need to include the word for 'you' (ti) before the verb?

No, you do not. Just like in English where we say 'Take the jacket' instead of 'You take the jacket', Croatian normally drops the pronoun in commands. The verb form uzmi already makes it perfectly clear who you are talking to.