"Those bridges which are between the hills are big."
Translation:Azok a hidak nagyok, amelyek a hegyek között vannak.
I seem to have a block here: why is the vannak at the end of the sentence and not after nagyok and in front of the comma? The sentence before is: Az a könyv van az asztal alatt, amelyik nem híres. And here the 'van' is in the first part of the sentence. I hope somebody can explain the difference.
I don't know if it'll help you or not, but here's one way to think about sentences with this kind of az ... ami or az ... amelyik or similar constructions. View the the first part of the sentence as a sentence which is gramatically complete in its own right, but doesn't convey enough specific information yet. So: Azok a hidak nagyok. is a gramatically complete sentence, but the word azok leaves the listener wondering, WHICH bridges? So you clarify that with a clause that answers the question, "which bridges?"
Likewise, Az a könyv van az asztal alatt is a gramatically complete sentence, but the words Az a könyv (especially right before the verb like that) leave the listener really wanting to know something: "What do you mean, 'that' book? Which book are you talking about?" So the clause follows to answer that question. And the clause is also a complete normal sentence except its subject is omitted (since it is already present in the first part of the sentence), and it has the coordinating conjunction instead.
The first part of the sentence doesn't need a verb, since it's 3rd person and just has an adjective as its predicate: Azok a hidak nagyok.
The clause in the second part is talking about where the hills are (exist), so it needs the "existence verb": [Azok], ameleyek a hegyek között vannak
In your other example, the first part of the sentence is talking about something existing in a place, and it's specifically trying to tell you WHAT it is that is under the table. So it needs van, and it needs the book to come right before the verb: Mi van az asztal alatt? Az a könyv van az asztal alatt.
The second part of the sentence goes on to describe it using just an adjective, which doesn't require a verb. Melyik könyv? Az, amelyik nem híres.