As mentioned above, 吗 is a question particle that is used to turn statements into yes-no questions. What exactly does that mean? A yes-no question is also known as a “binary question” or a “polar question”. This simply means that it’s a question that can only be answered with “yes” or “no”. In other words, it’s not an open question.

The question particle 吗 is the easiest way to form this kind of question in Mandarin Chinese. All you do is put it on the end of a plain statement, and the statement becomes a yes-no question. Have a look at some examples:

Nǐ shì Lǐ Xiānshēng ma?
Are you Mr Li?
Nǐ huì Zhōngwén ma?
Do you speak Chinese?
Zhèli yǒu xǐshǒujiān ma?
Is there a toilet here?
Those would all be valid sentences without 吗. They would just be plain statements:

Nǐ shì Lǐ Xiānshēng.
You are Mr Li.
Nǐ huì Zhōngwén.
You speak Chinese.
Zhèli yǒu xǐshǒujiān.
There is a toilet here.
Compare the two sets of sentences. You can see that when 吗 is added on the end, they become yes-no questions. All of those questions can only be answered with agreement or disagreement. That’s what 吗 is for.

In this way, 吗 is almost like a question mark that you say out loud. It goes on the end of the sentence and indicates that it’s a question. Have a look at some point statements being changed into yes-no questions with 吗:

Zhè shì nǐ de.
This is yours.
Zhè shì nǐ de ma?
Is this yours?
Tā shì Tàiguórén.
He's from Thailand.
Tā shì Tàiguórén ma?
Is he from Thailand?
Nà shì fēijī.
That's a plane.
Nà shì fēijī ma?
Is that a plane?
Notice how in English you have to re-arrange the word order of the sentence to form these questions. In Chinese, all you have to do is add 吗 on the end.