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:

你是李先生吗?
Nǐ shì Lǐ Xiānshēng ma?
Are you Mr Li?
你会中文吗?
Nǐ huì Zhōngwén ma?
Do you speak Chinese?
这里有洗手间吗?
Zhèli yǒu xǐshǒujiān ma?
Is there a toilet here?
Those would all be valid sentences without 吗. They would just be plain statements:

你是李先生。
Nǐ shì Lǐ Xiānshēng.
You are Mr Li.
你会中文。
Nǐ huì Zhōngwén.
You speak Chinese.
这里有洗手间。
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:

这是你的。
Zhè shì nǐ de.
This is yours.
这是你的吗?
Zhè shì nǐ de ma?
Is this yours?
他是泰国人。
Tā shì Tàiguórén.
He's from Thailand.
他是泰国人吗?
Tā shì Tàiguórén ma?
Is he from Thailand?
那是飞机。
Nà shì fēijī.
That's a plane.
那是飞机吗?
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.