“Do you know if it's mandatory?”

There's a meeting scheduled for this afternoon. You're busy with other work and don't really want to go. You're not sure whether attendance is required at this meeting. When a coworker asks if you're going, you ask:

Do you know if it's mandatory?

Do you know if (clause)?

In casual spoken English, people often ask, "Do you know if ___?" For example:

Do you know if you're having a boy or girl?

Do you know if he's awake?

In formal English for writing and for situations where you don't know people well, you usually use "whether" instead of "if":

Do they know whether the child is a boy or a girl?

(something) is mandatory

When you say that something "is mandatory", it means that you have to do it. Examples of things that are "mandatory" include:

• a mandatory meeting
• mandatory reading (books you have to read for a class)
• mandatory retirement (making people retire when they reach a certain age)
• mandatory military service (men in some countries have to serve in the military for a few years)