Learn Hardcore Turkish: Ben arkadaşa mektup yazıyorum.

The word arkadaşa includes the dative case suffix -a (or -e after vowel harmony), which indicates the direction or recipient of the action. In this case, it translates to "to a friend", showing that the friend is the indirect object of the sentence.

Why is there no accusative ending on mektup?

In Turkish, the accusative case marker (such as -ı, -i, -u, -ü) is used to mark definite direct objects. Since mektup here means "a letter" (an indefinite object), it doesn’t take the accusative suffix. The absence of the suffix tells us that the letter is not a specific, previously identified one.

How is the present continuous tense formed in yazıyorum?

The verb yazmak (to write) combines with the present continuous suffix -ıyor (adjusted for vowel harmony) and the first person singular ending -um. Putting these together—yaz (stem) + -ıyor (present continuous marker) + -um (first person ending)—gives you yazıyorum, which means "I am writing."

Is it necessary to include the pronoun Ben when the verb ending already indicates the subject?

No, it is not strictly necessary because Turkish verb conjugations clearly indicate the subject through their endings. However, including Ben can add emphasis, clarity, or contrast, especially if the speaker wants to explicitly stress who is carrying out the action.

What is the word order of this sentence compared to English?

Turkish typically follows a Subject-Object-Verb (SOV) order. In Ben arkadaşa mektup yazıyorum, Ben is the subject, arkadaşa mektup forms the object part (with arkadaşa as the indirect object and mektup as the direct, indefinite object), and yazıyorum is the verb. In English, this sentence is rearranged to a Subject-Verb-Object order: "I am writing a letter to a friend."