Questions & Answers about Onko tämä isän numero?
Why does isä change to isän here?
isän is the genitive (possessive) form of isä (father). By adding the -n ending, it acts just like the English "'s" and translates to "father's". It tells us whose number it is.
Why does the sentence start with onko instead of just on?
on means "is". To turn a regular statement into a yes/no question, Finnish attaches the question ending -ko to the verb and moves it to the very beginning of the sentence. So, onko directly translates to "is?".
The English translation says "the father's number", but where is the word for "the"?
Finnish doesn't have articles like "a" or "the". A phrase like isän numero can mean "a father's number" or "the father's number" depending entirely on the context of the conversation.