"Do you think these are plates?"
Translation:Ты думаешь, это тарелки?
These plates are blue: эти тарелки синие. These are blue plates: это синие тарелки.
If you know any French, there's actually a basically identical construct that may help clarify:
These plates are blue: Ces assiettes sont bleues. These are blue plates: Ce sont assiettes bleues.
I'm still a beginner in Russian, but have experience in other languages. By using это, you are making "this" the subject of the second clause, instead of having it be a determiner meaning "these plates". Since it is a subject, the verb to be is implied, so that it ends up being clear that you are referring to an abstract object [e.g., "this stuff"], and asking whether the plates qualify as members of the abstract object's class of things: "Do you think this [stuff] is/are plates?"
If "this" were to agree with "plates" эти тарелки, then there would be no verb in the second clause, turning the sentence into something like "Do you think of these plates", which doesn't make any sense unless you add "what" что, AND you'd have to change the case of "these plates" to (probably) prepositional.
I believe its because of the word Как which translates to How. Since Russian doesn't have the word "Do" its just always implied in a question. If you use it then the sentence meaning would be changed. Heres some examples -http://masterrussian.com/vocabulary/kak_how_what.htm
My understanding is that you use это when you are not really describing the object being referred to, as in, "This is a plate", and you use the" Singular masculine, feminine, plural " forms when you are referring directly to the object with description "this apple is tasty" this plate is big, etc...