Learn Turkish: Bebeğin kıyafetleri yıkanmalı. - The baby's clothes must be washed.

The root verb is yıkamak (to wash). Because it ends in a vowel, it takes the passive suffix -n to become yıkanmak (to be washed). Finally, the necessity suffix -malı is added to mean "must be washed".

Why is it bebeğin with a ğ instead of a k?

The word for baby is bebek. When we add the genitive suffix -in (to mean "the baby's"), the hard k at the end of the root softens to a ğ. This is a standard consonant mutation rule when a vowel suffix is added.

Why does kıyafetleri end with an -i if it is already plural?

The -i is the possessive suffix. To say "the baby's clothes", Turkish uses a genitive-possessive pair. Bebeğin has the genitive suffix (-in), and kıyafetler (clothes) takes the 3rd-person possessive suffix (-i) to complete the link.

If I wanted to say who must wash the clothes, how would I add it to this passive sentence?

Since yıkanmalı is passive ("must be washed"), you would use the word tarafından (by) to specify the doer. For example, Bebeğin kıyafetleri babası tarafından yıkanmalı (The baby's clothes must be washed by his father).