Learn Swedish: Turisten bytte resväskan i butiken. - The tourist exchanged the suitcase in the store.

The infinitive is byta (to exchange or switch). It is a Group 2b verb, which means its stem (byt-) ends in a voiceless consonant (like p, t, k, or s). Because of this hard consonant, you add -te instead of -de to make it past tense, resulting in bytte.

Resväskan is a long word. How is it built?

It is a compound word made from resa (trip or journey) and väska (bag). Together, a resväska is a suitcase. The -n at the very end makes it definite, meaning "the suitcase".

Could I say på butiken instead of i butiken?

It is much better to use i (in) here. Swedish uses i with butik (store) and affär (shop) to emphasize being physically inside the enclosed space of the building. While you use på for certain public locations like a bank or train station, stores generally take i.

Why do almost all the nouns in this sentence end in -en or -n?