Learn Finnish: Haastattelu on valmis. - The interview is ready.

It is built from the verb haastatella (to interview). In Finnish, you can often create a noun describing an activity by adding a -u or -y ending to the verb's strong stem.

Why does haastattelu have a double tt, while the verb haastatella only has a single t?

This is a great example of consonant gradation! Haastatella is a Type 3 verb. Type 3 verbs have the weak consonant grade in their basic dictionary form (single t), but they flip to the strong grade when conjugated (double tt). The noun haastattelu is built from that strong stem.

Since this lesson is about the past tense, how would we say "I interviewed" using that strong stem?

First, take the strong present stem: haastattele-. To make it past tense, change the final -e- to the past tense marker -i-, and add the "I" ending -n. This gives you haastattelin (I interviewed). The double tt stays strong.

Can valmis mean both "ready" and "finished"?

Yes, exactly! In Finnish, valmis can mean "ready" (prepared to start) or "finished" (completed). You have to rely on the context of the conversation to know whether the interview is just about to begin or if it is finally over.