Learn Russian: Директор не скрывал намерения купить эту компанию. - The director didn't hide the intention to buy this company.

Because of the negated verb не скрывал (didn't hide). In Russian, when the direct object of a transitive verb is negated, it frequently takes the genitive case instead of the accusative. This is called the genitive of negation. Here, намерение (neuter, nominative) becomes намерения in the genitive.

Since there is a не in the sentence, why isn't эту компанию also in the genitive case?

The negation only directly affects the verb скрывал (hid) and its direct object, намерения. The phrase эту компанию is the direct object of the infinitive купить (to buy), which is not negated. Since the director actually intended to buy the company, купить takes the standard accusative case.

Why does this sentence use the imperfective скрывал but the perfective купить?

Не скрывал (didn't hide) is imperfective because it describes a continuous state or attitude in the past—he was open about his plans over a period of time. Купить (to buy) is perfective because it refers to a specific, one-time action that he intended to achieve.

I know neuter nouns take -а or -я in the genitive. Why does намерение end in -ия?