Continuing from Kenny's answer: 1. "Make common cause (with someone)" is right. Examples here : http://fraze.it/n_search.jsp?q=make+common+cause&l=0&t=0&ffo=false&findid=-1&ff= "have common cause" is not a usual collocation of native speakers, in my experience and it would not have the same meaning as "make common cause". However, I did find one example of its use here (4th example): http://fraze.it/n_search.jsp?q=have+common+cause&l=0 2. OK 3. Depends on situation. "is treating me (now)" "treats me (generally)" Present simple is more common in practice but the situation determines the tense. 4. Your sentence is good. Kenny's additional "that" is optional.

To succeed ,we must have a common cause. What's the purpose of your visit to this country? I hate the way I look. >ok I like the way she is treating me.( she treats me) It's rather difficult to approach the girl you love and tell her that you love her.