with

[luoruosi]

I just realized I don't know how to use the word "with" in a description of someone in Mandarin, for example I want to say, "the girl who i used to work with".
would it be, "我从前跟她工作的那一个女人"?
I know I could use 同事 but that wouldn't work in every example, (the guy I used to fight with, the girl I go to the park with, etc.)

 

[ElGarcia]

Sometimes 一起 helps.

the girl who i used to work with 我以前一起工作的/以前和我一起工作的那个女人, or 那个以前跟我一起工作的女人
the girl I go to the park with 和我一起去公园的女孩

I'm not sure what exactly you mean by "the guy you used to fight with", you fought with him (you two were rivals) or you fought someone or something else together?

 

[luoruosi]

很容易啊，多谢，我明白了。

 

[ElGarcia]

Note that (you are probably already aware of this) "with" doesn't always mean 一起。
Like your other example: the guy I used to fight with --以前总和我打架的那小子 (the intuitive meaning, I used to fight with that guy)

 

[brofeelgood]

I'd say 一起 is more "together" than "with". It suggests collaboration. Its absence suggests interaction (either unilateral or bilateral) is involved.

- 跟我玩 - play with me
- 跟我一起玩 - play together with me

跟/和 would be closer to the meaning of "with".

- 和我说话的那个女人 - the woman who spoke with me/spoke to me
- 跟我打架的那个人 - the person who fought me/fought with me

 

[ElGarcia]

"With" is used in a wide range of circumstances.

- 和我说话的那个女人 - the woman who spoke with me/spoke to me
- 跟我打架的那个人 - the person who fought me/fought with me

These definitely fall into the category where I wouldn't use 一起.

However sometimes "with" does imply "together", where "together" may even be omitted. "Play with me" and "play together with me" mean pretty much the same thing under such context, and both 跟我玩 and 跟我一起玩 are fine. In the OP's first example, 跟我工作 doesn't sound complete in Chinese though. Since you can clearly tell this "with" suggests "you two work together", 跟我一起工作 would be more appropriate.