Learn Turkish: Lütfen kapıyı kapatma! - Please do not close the door!

Because the door is the specific object being closed. In Turkish, when an action happens to a specific thing, we add the accusative suffix to it. Since kapı ends in a vowel, we add the buffer letter y before the suffix ı, making it kapıyı.

Why doesn't the verb end in -mak?

Dictionary verbs end in -mak or -mek, like kapatmak (to close). When giving a direct command to someone informally, we drop the -mak to find the verb root. So the command 'close!' is just kapat, and the negative is kapatma.

How does the word kapatma mean 'do not close'?

The root of the verb is kapat (close). To make a command negative, we add the suffix -ma or -me to the root. Because kapat has back vowels, we use -ma, creating kapatma (do not close!).

Is kapatma directed at one person or multiple people?

It is directed at one person informally. Just the root plus the negative suffix makes it a singular, informal command. If you were speaking formally or to a group of people, you would add a personal suffix and say kapatmayın.