Learn Turkish: Merhaba, kolay gelsin! - Hello, may your work go easily!

Kolay means easy, and gelsin is a form of the verb gelmek (to come) that means may it come or let it come. So literally, it translates to may it come easy.

When do Turkish speakers actually use the phrase kolay gelsin?

You use it anytime you see someone working, studying, or doing a task. It is a very polite and common way to acknowledge someone's effort. You can say it to a cashier when you enter a store, a street sweeper, or a friend doing their homework.

Why is the verb gelmek (to come) used here instead of a word for work?

It is a set Turkish idiom. Instead of literally saying your work should be easy, Turkish expresses this wish as a hope that the task approaches or comes easily to the person doing it.

Is gelsin a command? I thought commands were only used for giving direct orders to people.

Yes, it is the third-person command form of gelmek. While in English we mostly give commands to the person we are talking to, Turkish has command forms for he, she, and it as well. It translates best as let it or may it. As a beginner, you do not need to memorize this grammar rule yet, just learn the phrase as a whole!