Learn Finnish: Vene liikkuu paljon, kun tuuli on kova. - The boat moves a lot when the wind is strong.

In Finnish, you almost always use a comma to separate the main sentence (Vene liikkuu paljon) from a subordinate clause that starts with a word like kun (when), että (that), or koska (because).

Both kun and milloin mean "when". Why do we use kun here?

kun is a conjunction used to connect sentences, like "when" in this statement. milloin is a question word used to ask "when?", like in Milloin vene liikkuu? (When does the boat move?). You cannot use milloin to connect a subordinate clause.

Why does the verb liikkuu end with two u letters?

liikkuu comes from the verb liikkua (to move). To make the "he/she/it" form (third person singular) in the present tense, you take the stem liikku- and double the last vowel, which makes it liikkuu.

I thought kova meant "hard". Why is it translated as "strong" here?

kova literally means "hard", but in Finnish, it is also used to describe intense weather or conditions. So a "strong wind" is kova tuuli, a "heavy rain" is kova sade, and a "loud noise" is kova ääni.