I am wondering how can we use "can't help it" in a sentence. I want you to explain me it's grammer definition and I would appreciate if you could provide some examples of this expression for me
"can't help it" is, as a phrase, roughly synonymous with "unable to prevent it from happening/being the case" where "it" is an action or habit previously or about to be mentioned.
Here are some examples of its use:
I can't help it if you don't have enough money! (I am unable to prevent the situation where you have insufficient funds for your purchase)
Biting strangers isn't his fault; he can't help it. (He is unable to prevent his compulsion to sink his teeth into people he doesn't know)
Alternatively, click Max Williams' link.
This specific sense of help usage is defined by OED as...
help 11a: To remedy, obviate, prevent, cause to be otherwise.
(With can, cannot, or some equivalent.)
It goes back a long way - two of OED's early citations are...
Note that this usage always requires a negatory context. It's perfectly normal to say I can't help [it, or "doing something"] to mean I'm unable to stop [doing] it, but no-one ever says I can help it to mean they are able to prevent something happening.
Common usages today include...
He can't help it ("it" is usually something undesirable that he can't stop doing)
It can't be helped ("it" is some undesirable situation that cannot be avoided)
He couldn't help laughing (his undesirable action expressed using the continuous verb form)