Sentence question

Are these sentences correct?

Yes.

Thanks. How about it I said:

I have not been to the gym for a long time.
I have not been to the gyn since a long time.

Is since used correctly?

AnonymousIs since used correctly?

No. You need a fixed date or time such as:
since 2009.
since January.
since August 8.

I see. What about the first one, with for a ling time? Is it correct?

Also, can I say since two years in addition to the fixed dates and times?

AnonymousI see. What about the first one, with for a ling time? Is it correct?

Yes.

AnonymousAlso, can I say since two years in addition to the fixed dates and times?

No. "two years" is a time span, not a fixed time.

Thank you. Is there an explanation as to why the time span is not possible but a fixed time is?

Why is since here possible: it has been 3 years since we last met.

But not possible here: I have not worked out since 3 years.

Any explanation on the time span versus the fixed time?

Anonymousit has been 3 years since we last met.

"we last met" defines a point in time. In this case, 3 years before the current date - that is November 11, 2011 is the date you met the other person.

AnonymousBut not possible here: I have not worked out since 3 years.

3 years cannot be resolved to a date, since there

Thanks you. So it is just that there has to be a point in time with the use of 'since', and that is just the way it works. Have I understood correctly?

AnonymousThanks you. So it is just that there has to be a point in time with the use of 'since', and that is just the way it works. Have I understood correctly?

Yes.
See definition #2 http://dictionary.reference.com/browse/since?s=t