Learn Finnish: Katolla on lintu. - There is a bird on the roof.

This is how Finnish says "there is" or "there are" something somewhere. While English starts with "There is...", Finnish usually starts with the location (katolla), followed by the verb (on), and finally the thing that is there (lintu).

How do we know it translates to "a bird" and not "the bird"?

Finnish doesn't have words for "a" or "the", so the meaning often comes from the word order. Starting with the location introduces new information, which translates best as "a bird". If you wanted to say "the bird is on the roof", you would use normal word order: Lintu on katolla.

Why does katolla end in -lla?

The ending -lla is the Adessive case, which usually translates to "on" or "on top of". If you used the Inessive case ending -ssa, it would mean the bird is inside the roof!

The dictionary word for roof is katto. Why does it only have one t in katolla?

This is due to consonant gradation, a spelling rule you'll see often. Adding the -lla ending "closes" the final syllable, causing the "strong" double consonant tt in katto to weaken into a single t.