How can I use by? (I will be there by 8:00 pm o, clock.)

[Nasimsuperb]

i will be there by 8:00 pm o,clock.
I will go there by sunday.
i will be there by 3:00 pm.

Is these all sentences are correct?
because confused about use of "by".
could anyone make me clear use of "By" please.

[Fan Of Arabian Horses]

Good question nasim.....,

perhaps I can offer some clarification:

Any action that has been done "by ....." must have been done at the latest at the mentioned point of time, no matter how long it lasts finally.

Another thing is "till" : actions that has been done "till... " will finally end at the special point of time.

Thus your sentences are fine (grammatically) It depends on the context of them.
I will be there by 8.00 pm (no o´clock necessary) means you arrive there at 8:00pm at the latest.
I will be there till 8.00 pm means you leave the site/place/or whatever at 8:00 pm.

I hope I was able to give a correct explanation and enabled you to find the correct solutions to your both other sentences. Let´s see what teachers say .............teachers, please, don´t torture me for that trial of explanation.

Michael