correct my sentence please.

[backspace2062]

I lost my pen, I think someone took it with himself/herself accidentally.
Is this sentence correct? is it necessary to use himself/herself. IF I don't know the gender of person whom took my pen what should i use instead of himself/herself.
Thanks.

 

[MikeNewYork]

I lost my pen, I think someone took it with himself/herself accidentally.
Is this sentence correct? is it necessary to use himself/herself. IF I don't know the gender of person whom took my pen what should i use instead of himself/herself.
Thanks.

No, it is not correct. In general, one can only use a reflexive pronoun (himself, herself) if the person had been mentioned earlier.

She did it herself. OK
He took it himself. OK
Otherwise use the normal objective pronoun: I lost my pen, I think someone took it with him/her accidentally.

 

[Chicken Sandwich]

Is this sentence correct? is it necessary to use himself/herself. No. IF I don't know the gender of person whom took my pen what should i use instead of himself/herself.

You could also write, 'I think someone took it accidentally'.

 

[Raymott]

I lost my pen. I think someone took it with himself/herself accidentally.
Is this sentence correct? is it necessary to use himself/herself. IF I don't know the gender of person whom took my pen what should i use instead of himself/herself.
Thanks.

Note that no one would ever say "with him/her". I'm assuming you're talking about speech.
If "I think someone took it accidentally" is not enough, you can say, "I think someone accidentally took it with them".
If you are writing this for some reason, you might write, "I think someone must have accidentally taken it [with them]."

Also, note that you have a comma splice. You can't join two sentences with a comma in English.