Learn Turkish: Arılar saçlarıma girmesin diye şapkama tül taktım. - I attached a net to my hat so that bees don't get into my hair.

Girmesin is the third-person imperative form, meaning "let them not enter." The word diye comes from the verb demek (to say). So, girmesin diye literally translates to "saying 'let them not enter'." This combination of an imperative or optative verb followed by diye is the standard Turkish way to build a purpose clause (a "so that" clause).

Since the subject arılar is plural, why isn't the verb girmesinler?

In Turkish, when a plural subject is non-human (like animals, objects, or concepts), the verb typically remains in the singular form. Because bees (arılar) are animals, girmesin is the natural and grammatically correct choice here rather than girmesinler.

Why is the word for hair pluralized as saçlarıma here?

In English, "hair" is an uncountable noun. In Turkish, however, when you talk about the hair on your head collectively, it is very common to use the plural form saçlar. Here it takes the "my" possessive ending (-ım) and the dative case (-a), becoming saçlarıma (to my hair).

Both saçlarıma and şapkama end in the dative case (-a). Why?