Learn Finnish: Nuori mies on sodassa. - The young man is in the war.

The basic dictionary word for war is sota. When we add a double-consonant ending like the "in" suffix -ssa, it closes the syllable. This triggers consonant gradation, causing a single t to weaken into a d. That is why sota becomes sodassa.

Why do we use the ending -ssa instead of -ssä?

This is due to vowel harmony. The word sota contains the back vowels o and a. Because of this, any endings attached to it must also use back vowels, making the correct ending -ssa.

The English translation uses the word 'the' twice. Why aren't there any extra words for it in the Finnish sentence?

Finnish does not have definite articles (like 'the') or indefinite articles (like 'a' or 'an'). The phrase nuori mies can mean 'a young man' or 'the young man', and sodassa can mean 'in a war' or 'in the war'. You simply figure out which one makes the most sense based on the context.

Why don't we add any special endings to nuori (young)?

Nuori describes the subject of the sentence, mies (man). Because the subject is just performing the verb and is in its basic dictionary form, the adjective describing it also stays in its basic form. In Finnish, adjectives only get extra case endings if the noun they are attached to gets an ending.