Learn Russian: Оставьте сумку здесь.

Because this is a request for a single, specific action to be completed. In Russian, we use perfective verbs in the imperative (command) form when we want someone to do something once and get it done, rather than a repeated action or general instruction.

Why does the word for bag change to сумку?

Сумка is the direct object of the sentence because it is the thing being left. This means it must be in the accusative case. For feminine nouns ending in -а, the accusative ending changes to -у.

Why does the verb end in -те?

The -те ending shows that this command is formal or plural, matching the pronoun вы (you). If you were speaking informally to a friend or family member (ты), you would drop the -те and just say оставь.

Could I say сюда instead of здесь for 'here'?

No, you need здесь because it indicates a static location (where the bag will stay). You use сюда (to here) with verbs of motion or placement, like положите (put). However, the verb оставить (to leave) focuses on the object's resting place, so it requires здесь.