Learn Finnish: Vanha mies on kirkossa. - The old man is in the church.

This is because of consonant gradation. When we add the ending -ssa to kirkko, it closes the final syllable (making it sound like "kos-sa"). In Finnish, closing a syllable often weakens the consonants right before it. Here, the double kk simplifies into a single k, giving us kirkossa.

What does the -ssa ending do?

The -ssa ending (known as the inessive case) translates to "in" or "inside" in English. By attaching it to the end of a noun, you change kirkko (church) to kirkossa (in the church).

Why doesn't the adjective vanha (old) get the -ssa ending too?

Adjectives always have to match the ending of the noun they describe. In this sentence, vanha describes the man (mies), not the church. Since mies is just in its basic, dictionary form, vanha stays in its basic form too. If you wanted to say "in the old church," it would be vanhassa kirkossa.

There is no word for "the" in this sentence. How do we know it's "The old man" and not "An old man"?

Finnish doesn't have articles like "a", "an", or "the". You usually figure it out from context or word order. When a sentence starts with the subject like this (Vanha mies on...), it generally refers to a specific, known subject, making "The old man" the most natural translation.