Learn Finnish: Iso puu on metsässä. - The big tree is in the forest.

This is because of vowel harmony! Finnish words that contain the front vowels ä, ö, or y take the front vowel version of endings. Since metsä contains an e and an ä, it takes -ssä.

Did the word metsä change at all when adding the ending?

No! Metsä has a non-gradating stem, which means its core shape doesn't undergo any consonant changes when you add an ending. You simply attach the -ssä directly to the basic word.

There is no word for "the" in this sentence. How do we know it's "The big tree" and not "A big tree"?

Finnish doesn't have articles like "a" or "the". Instead, word order gives us a clue. When a sentence starts with the subject like Iso puu (Big tree), it usually refers to a specific thing we already know about, so we naturally translate it as "The big tree" in English.

Why doesn't the adjective iso get an -ssa or -ssä ending too?

Adjectives in Finnish must match the noun they describe. Here, iso (big) describes the tree (puu), which is in its basic dictionary form. If the sentence instead meant "The tree is in a big forest", the adjective would have to match the forest, becoming isossa metsässä.