Learn Swedish: Häxan flög ut ur fönstret. - The witch flew out of the window.

ut shows the direction of movement ("out"), but on its own it doesn't mean "out of". To specifically describe moving from the inside of an enclosed space to the outside, Swedish pairs it with ur. Saying just flög ut fönstret would sound grammatically incomplete, much like saying "flew out window" in English.

This lesson covers stem alterations. Is that why fönster becomes fönstret?

Exactly! The base word is fönster (an ett word). When adding the definite suffix -et, the stem drops its final e to make it easier to pronounce. So it becomes fönstret rather than the clunky fönsteret. This type of stem alteration is very common for nouns ending in -er, -el, or -en.

What is the infinitive form of flög, and why doesn't it have a regular past tense ending?

The infinitive is flyga (to fly). It is a strong verb, meaning it changes its core vowel in the past tense instead of taking a regular ending like -de or -te. The vowel shift from y to ö is a pattern you will see in other irregular Swedish verbs too, such as frysa to frös (freeze to froze).