Translation:Those bridges which are between the hills are big.
The "az a konyv hosszu" sentence translates "az a" as simply the. Now, just because it's plural, all of a sudden it means those. Can we get some consistency, please?
Same remark: some sentences with the similar construction (azok az) are translated with "the" and other with "those" sometimes "the" is accepted, sometimes rejected. I fully agree with Arcaeca that this course is currently full of inconsistencies. This may explain why it takes such a long time to correct them. Let's be patient.
"Those bridges between the hills are big" would convey the exact same meaning. This translation is unnecessarily pedantic
sometimes "az a" is that or the. Sometimes "azok a" is those or the. And if we use the other one is wrong :(
I really don't understand this one... I wrote " Those big bridges are between the mountain" and i don't know why this isn't correct...
That would be
Azok a nagy hidak a hegyek között vannak.
The difference here is that the statement above is more a differentiator. It implies there are more bridges that we could be talking about, but the ones between the mountains are the ones we are describing as big. Your statement implies that those big bridges are the only ones we could be talking about. The use of "amelyek" is a big deal here.
Your understanding is absolutely correct. The whole exercise here is built around a nuance meaning to emphasise 'that one single specific' versus 'some other unspoken one that is different' and could be understood from the context. As a native speaker I'd never use this structure. Very arbitrary and subjective.
Can this statement be read as being about all bridges between hills (i.e. the "a" is akin to the grammatically mandatory ones in general statements)?
Nope. The use of 'azok a' here is very specific. You would need 'minden' in there to infer what you're asking about.
How would you say in Hungarian "THE bridges that are between the mountains ..."?
If only the lessons were consistent in requiring the or those when az/azok are used. I've seen "those" rejected when azok was used, and now I've had "the" rejected in the same place.
amelyik / amelyek - both words mean which so is this a case of sound harmony? if yes which sound?
That would be: "A hidak, amelyek a hegyek között vannak...". But, as a native speaker, I would avoid such a descriptive structure, and lean towards the compressed ones: "A hegyek között levő hidak..." or even: "A hegyek köz(öt)ti hidak..."
The part in parentheses can be freely added or omitted, as stylistic variants. So, "köz(öt)ti" means both "közti" and "közötti".
"közötti" means exactly that: "someone/something that is between". The Hungarian language uses those compressed structures quite a bit.