Learn Finnish: Minä odotan hetken. - I will wait a moment.

When you state how long an action lasts, Finnish often adds an -n to the time word. This is why the basic word hetki (a moment) becomes hetken (for a moment).

How do we know this means "I will wait" instead of "I am waiting"?

Finnish does not have a separate future tense, so the present tense is used for both what is happening now and what will happen in the future. Minä odotan can mean "I wait", "I am waiting", or "I will wait" depending on the situation.

Why is there only one "t" in odotan when the basic verb is odottaa?

This happens because of a rule called consonant gradation. When you add the -n ending for "I", the double tt in the middle of odottaa becomes a weaker single t.