Learn Turkish: Bir sene on iki ay. - One year is twelve months.

In Turkish, you never add the plural suffix (-lar or -ler) to a noun if a number comes right before it. Because we already said on iki (twelve), we just use the singular word ay.

Where is the verb for "is" in this sentence?

There isn't a separate word for "is" here. In Turkish, simple factual statements often don't need a verb. Just placing bir sene (one year) and on iki ay (twelve months) together naturally implies the "is".

I learned that the word for year is "yıl". Can I use that instead of "sene"?

Yes, absolutely. Sene and yıl mean exactly the same thing and can be used interchangeably. Bir yıl on iki ay would also be perfectly correct.

Does the word "ay" also mean "moon"?

That is correct! The word ay means both "month" and "moon". Just like in English, the Turkish concept of a month is tied to the cycles of the moon.