where there'll be lots of amateur bands ... ... où Il y aura plein de groups amateurs.

Question

Could you please explain why the future tense of être (to be) is incorrect when translating 'there'll be'.

Accepted Answer

I think I understand your confusion Stewart . 
The expression il y a has only one form the impersonal 'il' but it can be conjugated across the tenses and it uses the verb 'avoir' and not the verb 'être' as in English.

il y a = there is/are 
il y a eu = there has/have been 
il y avait = there was/were 
il va y avoir = there is going to be 
il y aura = there will be

etc.
Hope this helps!

Answer

‘Il y a’ is a very common fixed expression - ‘être’ cannot be substituted for avoir in this expression. This is not the only situation in which French uses ‘avoir’ for what in English would be the verb ‘to be’. That’s grammar for you !

‘Il y a..’ states what was/is/will/etc be (there). ‘Il/elle est …’ defines something about it.

As in English, you could say something more convoluted than the simple sentence given eg “… où Il y aura plein de groupes, et ils seront tous amateurs”. Not what you would normally hear in everyday speech though.

In the present tense - “Il y a une femme. Elle est grande.”

https://www.lawlessfrench.com/expressions/il-y-a/