Learn Turkish: On sekiz kız çalışkan. - Eighteen girls are hardworking.

It is very simple: you just combine the word for the tens and the units as two separate words. Here, on (ten) and sekiz (eight) are put together to make on sekiz (eighteen).

Why is it kız (girl) and not kızlar (girls) if there are eighteen of them?

This is a very important Turkish rule: when you put a number before a noun to say how many there are, the noun must always stay singular. Since on sekiz already shows there is more than one, saying on sekiz kızlar would be incorrect.

How does the sentence say 'are hardworking' without a word for 'are'?

Turkish does not use a separate word for 'am', 'is', or 'are' in the present tense. In simple statements like this, just placing the adjective çalışkan (hardworking) directly after the subject is enough to mean 'are hardworking'.