Learn Turkish: O asla sözünü tutmuyor.

The first -ü makes it 'his word' (possessive). The second -ü is the accusative case suffix, showing that 'his word' is the direct object of the verb. The -n- between them is a buffer letter that is always used when adding a case ending to a possessive suffix.

Asla means 'never'. Why is the verb tutmuyor negative as well?

In Turkish, words that mean 'never' like asla and hiç require a negative verb. Unlike English, Turkish requires this double negative to make the sentence grammatically correct.

The English translation uses the simple present 'keeps', but tutmuyor is present continuous. Why?

Turkish often uses the present continuous tense (-yor) to talk about habits, regular actions, and general behaviors, not just things happening right at this exact moment.

Why is the negative suffix in the verb -mu- instead of -ma-?

In the present continuous tense, the negative suffix -ma or -me narrows to -mı, -mi, -mu, or -mü before the -yor suffix due to vowel harmony. Because the vowel in tut is 'u', the negative suffix becomes -mu- (tutmuyor).