Learn Finnish: Räätäli ompelee puvun. - The tailor sews the suit.

Puvun is the total object (using the accusative case that looks exactly like the genitive). Using this case indicates a completed action: the tailor sews the entire suit and gets it done. If the sentence used the partitive pukua, it would mean the action is currently ongoing or incomplete ("The tailor is sewing a suit").

What is the dictionary form of the verb ompelee, and why does it look so different?

The infinitive is ommella (to sew), which is a Type 3 verb. Type 3 verbs have their infinitive in the "weak" consonant grade (here, mm). When you conjugate it to the third-person ompelee, it shifts to the "strong" grade (mp). This reverse consonant gradation can be tricky to spot!

What is the nominative (basic) form of puvun and what happened to the spelling?

The basic form is puku (suit). When we add the -n ending to make it the object, the word undergoes consonant gradation. Normally, a single k simply disappears in the weak grade, but there is a special rule: when a k sits between two u or y vowels, it changes to a v. That is why puku becomes puvun.

How do we know it means "The tailor" and not "A tailor"?

Because Finnish does not have articles like "a" or "the", räätäli can actually mean either. We translate it as "The tailor" here assuming it's the subject of a standard Subject-Verb-Object sentence, where the subject is usually already known from context. If we wanted to strongly emphasize "A tailor", we might change the word order or rely heavily on surrounding context.