Learn Finnish: Nuori nainen seisoo ulkona. - The young woman stands outside.

The dictionary form is seisoa, which is a Type 1 verb (ending in two vowels). To get the stem, we drop the final -a, leaving seiso-. For the third-person singular (he/she/it), Finnish doubles the last vowel of the stem. Since the stem ends in -o, we add another o to get seisoo.

Why is it translated as The young woman when there is no word for the in the Finnish sentence?

Finnish doesn't have articles like a or the. You usually tell them apart by word order and context. When a noun is the subject at the very beginning of a normal sentence like this one, it is usually talking about a specific person, so we translate it with The.

I have also seen the word ulos for outside. Why are we using ulkona here?

Both words mean outside, but they are used differently! You use ulkona when someone or something is resting or staying outside (location). You use ulos when someone is moving to the outside (movement). Since the woman is standing still in one place, we use ulkona.

If I wanted to say The young women stand outside (plural), would nuori change too?