38 If I do ... and If I did ...
Lisa: Shall we go by bus or by train?
JESS: If we go by bus, it will be cheaper.
if + do 表示可以发生的事情。
JOE:How are you going to travel?
JESS:We're going by train. If we went by bus, it would be cheaper, but the train is quicker.
if + past 用来表示我们认为不会发生，或者我们不希望发生的事情。但是不表示过去时。
一般would不出现在if那部分的句子中。比如：
If we went by bus, it would be cheaper. (not If we would go)
但是可以使用if ... would 去要求某人做某事。比如：
I would be grateful if you would let me know your decision as soon as possible.
另外一部分（非if的部分）我们一般用would和would't，could和might也可以用。
39. If I knew ... I wish I knew ...
If I knew his number, I would phone him.
If I knew 表示不知道号码。同样情况适用于I wish.
40. If I had known ... I wish I had known ...
我们用If + had表示过去。
I didn't see you when you passed me in the street. If I'd seen you, of course I would have said hello. (but I didn't see you)
would不写在if的部分，写在另外一个部分。
同理wish。
I wish I'd known that Gary was ill. I would have gone to see him. (but I didn't know)
不用在wish后面用would have.
The weather was cold when we were on holiday. I wish it had been warmer. (not I wish it would have been)
比较would (do) 和would have (done).前者表示现在时，后者表示过去时。
If I'd gone to the party last night, I would be tired now. (I am not tired now -present)
If I'd gone to the party last night, I would have met lots of people. (I didn't meet lots of people - past)
比较 would have， could have和might have：
If the weather hadn't bean so bad,
we would have gone out.
we could have gone out. (= we would have been able to go out)
we might have gone out. (= perhaps we would have gone out)
41. Wish
I wish you a pleasant stay here.
I hope you have a pleasant stay here. (not I wish you have)
我们可以说 wish somebody something，但是不能说I wish that something happends，在这种情况下我们要用hope.
用wish来表示后悔做某事。
I wish I knew what to do about the problem. (I don't know and I regret this)
用wish + had 来表示过去后悔做某事。
I wish I'd known about the party. I would have gone if I'd known. (I didn't know)
I wish I could (do something) 表示后悔做做不到的事情。
I'm sorry I have to go. I wish I could stay longer. (but I can't)
I wish I could have (done something) 表示过去.
I hear the party was great. I wish I could have gone. (but I couldn't go)
可以使用I wish (somebody) would (do something):
It's been raining all day. Tanya doesn't like it. She says: I wish it would stop raining.
使用I wish ...would用来抱怨某个事情：
The phone has been ringing for five minutes. I wish somebody would answer it.
I wish you'd do something instead of just sitting and doing nothing.
使用I wish ... wouldn't 来抱怨人们重复做的事情。
I wish you wouldn't keep interrupting me.
我们使用I wish ... would 来表示我们希望事情发生，但是我们不用它来表示how we would like things to be (这个没有理解)。比较：
I wish somebody would buy me a car.
I wish I had a car. (not I wish I would have)