Learn Russian: Она закрывает окно.

Because окно is a neuter, inanimate noun. In the accusative case (which we use for direct objects), inanimate neuter nouns do not change. They look exactly the same as their dictionary form.

Does закрывает mean 'closes' or 'is closing'?

It can mean both! Russian does not have a separate continuous tense like English does. The present tense verb закрывает translates to 'she closes' or 'she is closing' depending entirely on the context of the conversation.

Why does the first letter of окно sound like an 'a' when spoken?

This happens because of word stress. The emphasis in окно falls on the final syllable. In Russian, an unstressed о is pronounced like an 'a'. Since the first о is not stressed, it sounds like an 'a', while the stressed final о keeps its clear 'o' sound.

How do we conjugate the verb to get закрывает?

The dictionary form of this verb is закрывать. Because it ends in -ать, it follows the standard pattern for most verbs. To make it match она, you simply drop the -ть ending and add -ет.