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Sir,

I have not met him since 6 years ago.

I have not met him since long,long ago.

I have not met hims since a week ago.

I have not met him since a few days ago.

Are these sentences correct.

Thanks.
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You can only meet a person once, so these are not correct with "met" because they imply you are going to meet him (be introduced to him) again. You can, however, use "met with him" or "seen him". "since ... ago" is a little awkward. Here are some more idiomatic ways to say these:

I haven't met with him [ for six years / for a long, long time / for a week / for a few days ].
I haven't seen him [ for six years / for a long, long time / for a week / for a few days ].
I haven't spoken with him [ for six years ... (same) ].
I haven't heard from him [ for six years ... (same) ].

I haven't seen him since I met him six years ago.
I haven't spoken with him since I met him long ago.
I haven't heard from him since 1998.

CJ

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