Learn Polish: Trzeba smażyć cebulę i czosnek. - You must fry the onion and garlic.

Because smażyć is already a base imperfective verb. Suffixes like -iwać are typically added to perfective verbs (especially those with prefixes) to turn them back into imperfective verbs. Base verbs like smażyć naturally describe an ongoing process and don't need a special suffix to do so.

Could I use the perfective verb usmażyć after trzeba instead?

Yes, but it changes the focus. The imperfective smażyć emphasizes the ongoing process or action of cooking, which is very common in recipes and general instructions. If you used the perfective usmażyć, it would focus strictly on the completed result of having the vegetables fully fried.

Both the onion and garlic are direct objects being fried. Why does cebula change to cebulę, while czosnek doesn't change?

They are both in the accusative case, but they belong to different noun genders and follow different rules. Cebula is feminine, and feminine nouns ending in -a change to -ę in the accusative. Czosnek is a masculine inanimate noun, so its accusative form is identical to its nominative dictionary form.

How would I change this sentence to say 'I must fry the onion and garlic'?