Learn Swedish: Katten kröp in i lådan för att sova. - The cat crawled into the box to sleep.

The phrase in i means "into" and highlights movement from the outside to the inside of a specific container. If you used only i, it would mean the cat was already inside the box while crawling. If you used only in, you would usually drop the object, like katten kröp in (the cat crawled inside). Since we explicitly mention the box, we need both words to connect the motion to the object.

What is the base form of the verb kröp?

The infinitive is krypa (to crawl). It is an irregular, or strong, verb, which is why the vowel changes to an ö in the past tense. It is a good one to memorize at this level since the vowel shift isn't easily predictable!

Why do we need för att instead of just att to say "to sleep"?

In Swedish, whenever the English "to" actually means "in order to" or explains the purpose of an action, you must use för att. If you only wrote att sova, it would sound grammatically incorrect to a Swedish speaker because it drops the intention behind the cat's crawling.

Could I use the word kartong instead of låda here?