I ... wear a school uniform. It's dark blue.

Grammar Rule:

The phrase 'have to' is used to express obligation or necessity. The choice between 'have to' and 'has to' depends on the subject. 'Has to' is used with third-person singular subjects (he, she, it), while 'have to' is used with other subjects (I, you, we, they, plural nouns).

Solution:

The sentence begins with 'I', which is a first-person singular subject. Therefore, the correct form to express obligation is 'have to'.

• I have to wear a school uniform. It's dark blue.

Answer: have to