Learn Finnish: Onko paidassa reikä? - Is there a hole in the shirt?

Finnish doesn't use a filler word like "there" for "there is" or "there are" sentences. Instead, you usually start with the location (here, paidassa meaning "in the shirt") followed by the verb on (is) and the thing that exists. Since this is a question, the verb on moves to the front and takes the question ending -ko.

The dictionary word for "shirt" is paita. Why is it spelled with a 'd' in paidassa?

This happens because of a rule called consonant gradation. When you add an ending that closes the final syllable (like the ending -ssa, which adds two consonants), a single t in the middle of the word softens into a d. So, the strong grade paita weakens to paidassa.

How does the word onko work to make this a question?

On is the basic "he/she/it is" form of the verb olla (to be). To make a yes/no question in Finnish, you take the word you are asking about, move it to the very beginning of the sentence, and attach the question ending -ko or -kö. Therefore, onko simply means "Is [there]?"

If I wanted to say the negative—"There is no hole in the shirt"—how would that look?