1. if:

例：

If you do that again I'll hit you. （如果你再那么做，我就揍你！）

Do you think I'd be here if I had a choice? （你觉得我要是有得选的话会呆在这里吗？）

I have a drill. If you like, you can borrow it. （我有钻子，你要是想借，我可以借你。）

2. should: use this when something might happen in the future but it is not likely

备注：''should''常用于正式的场合。用法：虚拟语气"should sb done sth"或"should sth be done"或"should there be sth"

例：

We've planned everything very carefully, but should there be any problems, contact me immediately. （所有事情我们都安排好了，万一还有问题，马上跟我联系。）

Should you need any help, you can always phone me at the office. （如果你需要帮助，随时可以打电话到我办公室。）

3. had: use this when you are saying what the result would have been if things had happened differently in the past

备注：''had''常用于正式的场合。用法：虚拟语气"had sb done sth"或" had sth be done"或" had there be sth"

例：

Had I known earlier that you wanted to join the team, I'd have put your name on the list. （我要是早知道你想入队，我早就把你名字写上了。）

My horse would have won had he not fallen at the final fence. （如果我的马没在最后一栏摔倒，它早赢了。）

4. in case: use this to say that something is done because something else might happen or be true

例：

I'll take an umbrella in case it rains. （我带把伞，以防万一下雨。）

In case you missed the first episode, here is the story so far. （如果你之前没看第一季，现在是前情提要。）

In case you were thinking I'd lend you any money, I'll tell you now—I won't! （万一你觉得我会借钱给你那就不好了，我现在明白地告诉你，我不会借给你！）

just in case

I'm sure they haven't forgotten but let's send them a reminder just in case. （他们肯定不会忘记，但我们还是发个邮件给他们，提醒一下，以防万一。）

5. in case of: used especially in official notices and instructions to tell people what to do if something unpleasant or unexpected happens

备注：''in case of''常用于书面语中。

例：

In case of fire, leave the building by the nearest exit. （如果发生火灾，从离你最近的逃生出口逃出去。）

It is illegal to park on the hard shoulder except in case of emergency. （除非有紧急情况，否则把车停在硬质路肩上是违法的。）

6. in the event of something: use this when you are saying what will be done if at any time there is a serious problem, an accident etc - used especially in official notices, plans, or instructions

备注：''in the event of something''常用于正式场合，也可写作''in the event that something happens''。

例：

He left a letter for me to read in the event of his death. （他留了一封信给我，让我等他死后再拆阅。）

Britain agreed to support the US in the event of war. （英国答应若发生战争时就支持美国。）

The plan outlines emergency procedures in the event of a major accident. （这个计划草拟了发生严重事故时的一些急救程序。）

in the unlikely event of (=if something unlikely happens)

In the unlikely event of a burglar entering the building, the alarm system will be activated. （万一楼里进贼了，大楼的报警系统会发出警报。）