Learn Turkish: Anahtarı kullanıyor.

In this sentence, the key is the direct object of the verb kullanıyor (is using). Because it refers to a specific key (the key rather than just a key), it takes the definite accusative case suffix -ı.

How do we know to use -ı and not another version of the accusative suffix like -i or -u?

We look at the last vowel of the root word. The last vowel in anahtar is 'a'. According to 4-way vowel harmony rules, words whose last vowel is 'a' or 'ı' take the undotted -ı suffix.

There is no word for 'he' in this sentence. How do we know who is using the key?

Turkish often drops subject pronouns like o (he, she, or it). The verb kullanıyor ends right after the -ıyor tense suffix without any extra personal endings. In Turkish, a verb with no personal ending automatically refers to the third-person singular.

What part of kullanıyor makes it a continuous action?

The root of the verb is kullan (use). The -ıyor part is the present continuous tense suffix, which is very similar to '-ing' in English. It tells us the action is happening at this very moment.