Learn Turkish: Annem bir masal okumuş. - My mother read a fairytale.

The suffix -muş is the reported or inferential past tense. If you say okudu, you are stating a direct fact that you witnessed. By using okumuş, you mean 'Apparently, my mother read a fairytale' or 'I heard that she read a fairytale.' You would use this if someone else told you about it, or if you found an open book and guessed what happened.

How do we know to use -muş instead of -miş?

The reported past tense suffix follows 4-way vowel harmony (-mış, -miş, -muş, -müş). Because the last vowel in the verb root oku (to read) is u, the suffix changes to match it and becomes -muş.

Why is there no personal ending on the verb okumuş?

In Turkish, the third-person singular (he, she, or it) does not take a personal suffix. Since the subject is Annem (My mother), the verb just takes the tense suffix -muş without any extra letters at the end to show who did it.

Could I drop the word bir and just say Annem masal okumuş?

Yes, but it changes the focus slightly. Annem bir masal okumuş means she read a specific, even if unidentified, fairytale. If you drop bir, the noun merges with the verb, translating closer to 'My mother did some fairytale-reading,' where the focus is entirely on the activity itself.