Learn Turkish: Bu kalın kitabı yastık olarak kullandık.

When using olarak to mean "as" a role, function, or capacity, the noun before it stays in its bare dictionary form. So, "as a pillow" is simply yastık olarak, with no case endings needed on yastık.

Where does the word olarak actually come from?

It is built from the verb olmak (to be) and the adverbial suffix -arak (by doing / while doing). So, yastık olarak literally translates to "by being a pillow." Recognizing this structure will help you remember why it is used to express a role or capacity.

Why is the specific direct object (accusative) suffix used on kitabı?

The verb kullandık (we used) acts on the book. Because the sentence says "this thick book" (bu kalın kitabı), the book is a specific, known object. In Turkish, specific direct objects must take the accusative case suffix, which is the -ı at the end of the word.

Why did the "p" in kitap change to a "b" in kitabı?

This is a classic example of consonant mutation, often remembered as the KETÇAP rule. When a word ends in a hard consonant like p, ç, t, or k, and a suffix starting with a vowel is added (like the accusative -ı), the hard consonant softens. Here, the p in kitap softens into a b.