Learn Swedish: Hon tittar på dörren och fönstret. - She looks at the door and the window.

The basic word for "a door" is an en-word: en dörr. In Swedish, to say "the" for these words, you attach -en to the end of the noun. So, dörr becomes dörren.

Why is "the window" fönstret and not fönsteret?

"A window" is an ett-word: ett fönster. Usually, to say "the" for an ett-word, you add -et to the end. But when a word ends in an unstressed -er, the e before the r usually drops out when adding the ending to make it easier to say. So instead of fönsteret, it shrinks to fönstret.

Why do we use på here?

The word på means "on" or "at". In Swedish, the verb tittar is almost always followed by på when you want to say you are looking at something. So, tittar på together means "looks at".

Does tittar mean "looks" or "is looking"?

It means both! Swedish doesn't have a separate tense for things happening right now (like the English "-ing" form). Depending on the context, Hon tittar can be translated as "She looks" or "She is looking".