If the question was , "these seas are warm and shallow", would "ezek tengerek melegek és sékelyek" be the correct answer?
Ezek a tengerek. Don't forget the article. And it's sekélyek.
Why meleg and sekély are not in plurial form?
Because they are before the noun.
Is it wrong to include the definite article? "Ezek a meleg és sekély tengerek"?
It is not wrong, but only if you also do it in the English sentence:
"These are the warm and shallow seas."
what is wrong with "ezek tengerek melegek és sekélyek"?
"These are seas, warm and shallow ones." You forgot the comma, though. :B
If you have the adjectives in front of the noun in English, you also have them in front of the noun in Hungarian.
"Ezek meleg és sekély tengerek vannak". Is that wrong?
It is. You do not use van if you're describing the properties of an object.
Thanks, I thought only "van" wasn't used.
That rule accounts for both van and vannak (and conversely for nincs, nincsenek, sincs, and sincsenek). All other conjugation forms of van are included as normal.
What is wrong with "Ezek a tengerek meleg és sekély"?
The grammar is the wrong way around. You translated "These seas are warm and shallow." Also, if the adjectives are not in front of the noun, you need to pluralise them as well if the noun is pluralised: melegek és sekélyek.
ezek a tengerek melegek és sekélyek - not correct :(
That would translate to "These seas are warm and shallow". You are saying what these seas are like, but the original sentence wants to tell what "these" are.