Learn Finnish: Onko kello nyt kuusi? - Is it six o'clock now?

In Finnish, you can turn a statement into a yes/no question by adding the question particle -ko (or -kö) to the word you are asking about. Since on means "is", onko means "is it?".

Why does the sentence use the word kello (clock)?

To tell the time in Finnish, you literally talk about the "clock". While in English you say "it is six o'clock", in Finnish you say kello on kuusi (the clock is six).

Do I need to change the form of the number kuusi (six) when telling the time?

No! When saying it is exactly a certain hour, you just use the basic, dictionary form of the number. For example, "one o'clock" is kello on yksi and "six o'clock" is kello on kuusi.

Doesn't the word kuusi also mean something else?

Yes, it does! Kuusi means both the number "six" and a "spruce" or "fir tree". You can easily tell them apart from the context. Since this sentence is about a clock (kello), we know it means six!