Questions & Answers about Hän jää kotiin, siksi että sataa.
Why does it say siksi että instead of just koska for "because"?
Both mean "because", but siksi että (literally "for the reason that") is the direct pair to the question word miksi (why). If someone asks Miksi hän jää kotiin?, answering with siksi että feels very natural. You can also perfectly well use koska here.
Why is it kotiin (into home) and not kotona (at home) if he is staying there?
This is a famous Finnish quirk! While in English you stay at a place, the Finnish verb jäädä (to stay) requires the "into" form (the illative case). You can think of it as "he leaves himself into the home." Therefore, we must use kotiin instead of kotona.
Why is "it is raining" just sataa? Shouldn't there be a word for "it"?
No, Finnish does not use "dummy" pronouns for the weather like English does. You just use the verb all by itself. Saying se sataa (it rains) sounds unnatural to a Finnish speaker.
What is the dictionary form of the verb jää?
The dictionary form is jäädä (to stay / to remain). To make the hän (he/she) form, you usually take the stem and double the last vowel. Since the stem jää- already ends in a double vowel, no extra letters are added, making it just hän jää.