Learn Polish: Nie miałem takich zamiarów. Miałem tylko naprawić twój komputer. - I didn't have such intentions. I was only supposed to fix your computer.

When mieć (to have) is followed by a noun like zamiarów, it has its standard meaning of possession. But when it is followed by an infinitive verb, like naprawić in the second sentence, it takes on the meaning of an obligation or expectation. So miałem naprawić means "I was supposed to fix".

Why is it takich zamiarów instead of the basic plural takie zamiary?

This happens because of the negative word nie. The verb mieć normally takes the accusative case for its direct object. However, in Polish, whenever you negate a verb that takes the accusative, the object flips to the genitive case. Takich zamiarów is the genitive plural form of taki zamiar (such an intention).

Why doesn't the ending of twój komputer change, since it's the object being fixed?

It actually is in the accusative case here, but it doesn't look like it changed! Komputer is a masculine inanimate noun. For masculine inanimate nouns, the accusative case looks exactly identical to the nominative (dictionary) case. Therefore, both the pronoun twój and the noun komputer keep their basic forms.

Why use the perfective verb naprawić instead of the imperfective naprawiać?