Learn Hardcore Arabic (Modern Standard): عندما يرن جرس الباب، تذهب أمي إلى الباب وتفتح للضيفة. - When the doorbell rings, my mother goes to the door and opens it for the guest.

A useful thing to know is that عندما is commonly followed by a verb in the present tense in Modern Standard Arabic, even when English might translate it in a more flexible way.

You can think of the structure as:

عندما + present verb
When + something happens

It is very common in MSA narrative and formal writing.

Why is يرن in the present tense after عندما?

Because in Arabic, after عندما, the verb is often in the present tense to describe an action that happens at that time.

So:

عندما يرن جرس الباب = when the bell rings

Even if the whole sentence describes a repeated habit or a general situation, Arabic still naturally uses the present here.

Also, يرن is a 3rd person masculine singular verb because its subject is جرس and جرس is grammatically masculine singular.

Why is it جرس الباب and not الجرس الباب?

This is an iḍāfa construction, often called a genitive/possessive construction.

جرس الباب literally works like: