1) I am sorry but I just accepted another offer that I feel is more suitable to my needs / more right for me./ more interesting to me.
很抱歉，我刚接受了一个我认为更符合我需要的/更适合我的/更感兴趣的工作。
2) I really appreciate your invitation, but I must decline it.
非常感谢你们的邀请，但我不得不谢绝。
3) Frankly speaking, I have been thinking that the work mode is not right for us after the discussion.
坦白地说，讨论后我一直认为这个工作模式不适合我们。
4) I really think that position suits my education background better.
我真的觉得那个职位更适合我的教育背景。
5)I dont want to accept a position that I don’t think I will be happy in/offers me too little salary/ cant supply me with an apartment.
我不想接受一份我觉得不会让我快乐的/一份薪水太少的/不能为我提供住房的工作。