[Grammar] , but it not always works.

[atabitaraf]

This structure sounds familiar to me, but as I know the rules in grammar I couldn't make a reason for it.

"The first Newton formula is mentioned in mechanical physics, but it not always works."

 

[bhaisahab]

This structure sounds familiar to me, but as I know the rules in grammar I couldn't make a reason for it.

"The first Newton formula is mentioned in mechanical physics, but it not always works."

It's incorrect.