Learn Turkish: Benim oğlum kitap okuyor. - My son is reading a book.

When adding a suffix that starts with a vowel (like the possessive -um) to certain two-syllable words, the vowel in the second syllable drops out. This mostly happens with body parts and family members. So, oğul + -um becomes oğlum.

If oğlum already means "my son", why do we also need benim?

You don't strictly need it! In Turkish, the possessive suffix -um on oğlum is enough to show who the son belongs to. Adding benim just gives extra emphasis, like saying "My son is reading a book" as opposed to someone else's son.

Why does the sentence use kitap instead of bir kitap for "a book"?

In Turkish, when talking about an action generally, like "book-reading" or "apple-eating", you can just place the bare noun right before the verb without bir (a/an). Kitap okuyor translates naturally to "reading a book" or "reading books".

The verb stem is oku- (read). When adding the present continuous -yor, why doesn't the ending change like it does for other verbs?