The question particle 吗 (ma) is a very simple way to form questions in Chinese. By placing 吗 on the end of a statement, you convert it into a yes/no question. Surprisingly enough, these are questions that could be answered with yes or no in English. They’re also known as polar questions or binary questions.

Basic Pattern
1.Any statement can be converted into a yes / no question with 吗. For example:

It’s important to remember that you can not add 吗 to a sentence that’s already a question. For example:

These would be something like "Do you who are you?" and "Does is this a book?" in English – obviously wrong. Still if you’re not careful, you may find yourself throwing a 吗 onto the end of a question that doesn’t need it. Many learners make this mistake, so don’t worry if it happens every once in a while, just catch it and remember it the next time.

More Advanced Usage
However, this doesn’t mean that a sentence can’t ever have a question word and 吗. For example:

This is OK because the original sentence "你知道这是什么东西" is a statement even though it contains the question word 什么. It’s the same in English. "You know what this is" can be converted into the question "Do you know what this is?"