"Do you know where the Yangtze is?"

Translation: 你知道长江在哪儿吗？

[AidanGoodall]

Do you need 吗 here? Not accepted without

[NasuSamaruk0]

Yes, you do since a subject is being asked whether or not he/she knows about the location of the river. Since 吗 complements with the verb 知道 in place, the given translation is appropriate. Otherwise, you can use 知不知道

[DestinationVoid]

Could you provide any reference to this grammar structures?

[Ryf-Reto]

Why is 你知道长江在哪儿？not correct? As 哪 always is an interrogative form

[RajasDaithankar]

@Ryf-Reto - read the English question again. Where is NOT the interrogative in this instance.. Do you know? is the interrogative.. "Where" simply happens to be the subject of "Do you know"?

Now, "Do you know?" is a Yes/No question, and as you (hopefully) know, Yes/No questions need a 吗 or a X 不 X in the sentence construction.

Hope this helps.

[Ryf-Reto]

@Rajas - many thanks for your precision. Now I understand it clearly.

[RajasDaithankar]

My pleasure... Glad to be of help.