通常这些助动词不用于条件句中的if之后，然而以下情况例外。
A if you will/would常用于有礼貌的请求中，此时可用更有礼貌的形式would。
If you will/would wait a moment Ill see if Mr Jones is free.
请您稍候片刻，我看看琼斯先生是否有空。(请等候)
I would be very grateful if you would make the arrangements for me.
如果您能为我安排一下，我将不胜感激。
假如这种要求在当时的情况下通常都会提出，已成为套话，if you would+动词原形就常常单独使用。这表示说话人认为对方理所当然会照办。
If youd fill up this form.
请填好这张表。
(in a hotel) If youd just sign the register.
(在旅馆中)请在登记簿上签名。
(in a shop)If youd put your address on the back of the cheque.
(在商店里)请在支票的背面写下你的地址。
(in a classroom)If youd open your books.
(在教室里)请打开你们的书。
B 表示愿意的if+will/would适用于所有人称：
If hell listen to me Ill be able to help him.
如果他听我说，我就会帮助他。(如果他乐意听)
If Tom would tell me what he wants for his dinner Id cook it for him.
如果汤姆愿意告诉我他晚饭要吃什么，我就会做给他吃。(说话人的意思是说汤姆不愿告诉她。)如在这里使用wont时，则意味着拒绝：
If he won t listen to me I cant help him.
如果他不肯听我的话，我就不能帮他的忙。(如果他不愿听/如果他拒绝听)
If they wont accept a cheque well have to pay cash.
如果他们不愿接受支票我们就只能付现金。(如果他们拒收)
C Will 可以用来表示执意坚持(参见第230节B)：
If you will play the drums all night no wonder the neighbours complain.
如果你一定要整夜敲鼓的话，那就难怪邻居抱怨。(要是你执意敲的话)
D if+would like/care 可以用来代替 if+want/wish，而且比后者有礼貌：
If you would like to come Ill get a ticket for you.
如果你想来，我就给你弄一张票。
If youd care to see the photographs Ill bring them round
如果你有兴趣看那些照片，我就带来。
If hed like to leave his car here he can.
如果他想把车停放在这里，他可以这样做。
但是，如果改写一下句子，使 would like没有宾语，就可以省掉 would：
If you like Ill get a ticket for you.
如果你喜欢，我就给你弄一张票。
但是说：
If youd like a ticket Ill get one for you.
(译文同上。)
If he likes he can leave his car here.
如果他愿意，他可以把车停放在这里。
但是说：
If hed like to leave his car here he can./ He can leave it here
if hed like to.(译文同上。)
E if+should可以用于条件句类型1，用来表示动作虽有可能实现但可能性不很大。它常和祈使句结合使用，而且主要用于 书面的说明中：
If you should have any difficulty in getting spare parts ring this number.
万一你买备件时遇到什么困难，请打这个电话号码。
If these biscuits should arrive in a damaged condition please inform the factory at once.
饼干运到时如有损坏，请立即通知本厂。
should可以位于句首而省略掉if：
Should these biscuits arrive