Do this lyrics have mistake of grammars?

Not as bad as the thread title. Poetry isn't always structurally correct.

 

it's not horrible...but if you wanted it to sound like spoken fluent english it would read:

A million tomorrows shall (all) pass away,
do we forget (all) the joys we have today?

or something along those lines.

 

Its known as "Poetic licence"

 

I think it is "Ere we forget..."

See the dictionary.com definition:

ere ( P ) Pronunciation Key (âr)
prep.
Previous to; before.

conj.
Rather than; before

and your thread title should read:

"Do these lyrics have grammatical mistakes?"

or

"Does this lyric have a grammatical mistake?"