1.warn
If you warn someone about something such as 【a possible danger or problem】,you tell them about it so that they 【are aware of】 it.警告; 提醒
如：When I had my first baby friends warned me that children were expensive.
当我有了第一个孩子时,朋友们提醒我说养小孩是很花钱的.
2.alert If you alert someone to a situation,especially 【a dangerous or unpleasant situation】,you 【tell them about it】.向…发出警报
如：
He wanted to alert people to the activities of the group.
他想警告人们注意这个组织的行动.
注意1和2,warn只是警告可能会发生的情况,而alert是知道危险,发出的警报!
3.for the sake of 为了……起见.为了…的利益 【表示的是目的】
如：for the sake of relaxation 为了达到放松的目的
4.because of 由于 【只是解释原因】
如：He came back early because of the rain .
由于下雨,所以他回来得早.