Learn Turkish: Ülkeyi yönetenler siyasetçilerdir. - Those who govern the country are politicians.

The -dir suffix is the factual or generalizing copula. Here, it is used to state an objective fact or a general definition. If you just said Ülkeyi yönetenler siyasetçiler, it still means the same thing, but adding -dir makes it sound like an encyclopedic fact or a formal statement: "Those who govern the country are, by definition, politicians."

How does the word yönetenler mean "those who govern"?

It comes from the verb yönetmek (to govern). The -en suffix makes it a participle meaning "who governs" or "governing". When we want to say "the people who govern" without actually using the word "people", we just add the plural suffix -ler directly to it. So, yönet-en-ler literally becomes "the governing ones".

Why is it ülkeyi and not just ülke?

Ülke means "country". We add the buffer letter y and the specific direct object suffix (accusative case) -i because "the country" receives the action of being governed. It shows exactly what is being governed by the politicians.

What does the -çi do in the middle of siyasetçilerdir?