Learn Finnish: Oletteko te hädässä?

This is consonant gradation in action! The basic word is hätä. When we add the ending -ssä (meaning "in"), it closes the final syllable so that it ends in a consonant. This triggers a change in the consonant right before it: the single t weakens into a d.

Why is the ending -ssä instead of the usual -ssa for "in"?

This happens because of vowel harmony. The word hätä only contains the front vowel ä. Since Finnish words generally cannot mix back vowels (a, o, u) with front vowels (ä, ö, y), we must use the front-vowel version of the ending, which is -ssä.

Since oletteko already means "are you", do we really need the word te here?

No, it is optional! In Finnish, the verb ending (the -tte in olette) already tells us who is doing the action. Including the pronoun te just adds a little extra emphasis, similar to asking "Are you in distress?"

What is the -ko attached to the end of oletteko?

It is the Finnish question particle. To turn a regular statement like Te olette (You are) into a yes/no question, you take the main verb, move it to the very front of the sentence, and attach -ko (or -kö). This turns it into Oletteko te? (Are you?).