Learn Turkish: Saçları çabuk beyazlamasın diye her gün vitamin alıyor. - He takes vitamins every day so that his hair does not turn white quickly.

In Turkish, a common way to express purpose ("so that...") is by using the imperative or optative form of a verb followed by diye. Literally, the sentence is structured as "thinking 'let his hair not turn white quickly', he takes vitamins." Because the subject of the purpose clause ("his hair") is in the third person, the verb takes the third-person imperative suffix -sın.

Why is hair plural (saçları) when we usually treat it as a singular uncountable noun in English?

While saç can be used in the singular to mean hair in general, Turkish frequently uses the plural saçlar when referring to the collective mass of hair on someone's head. Here, saçları is made up of the root saç, the plural suffix -lar, and the third-person possessive suffix -ı meaning "his hairs".

Could I use için instead of diye in this sentence to mean "so that"?

Yes, but the grammatical structure would have to change. While diye pairs with the imperative or optative (beyazlamasın diye), için requires a verbal noun with a possessive suffix. To use için, the clause would become saçlarının çabuk beyazlamaması için. Using diye is very common, especially in spoken Turkish, because it avoids stacking too many suffixes.

How does the word beyazlamasın get its meaning of "turning white"?