英语中用if开头的句子结构就是用if开头句子的将来时和过去时的句子的结构

If 可以引导宾语从句表示“是否”的含义,也可以引导条件状语从句表示“如果”的含义.你要问时态的话那就要搞清楚它在引导条件状语从句时所用的虚拟语气的问题了.
a.与现在事实相反的假设
条件从句 主句
一般过去时（be用were） should（would）等 +动词原形
例如：If they were here,they would help you.如果他们在这儿,
含义：They are not here,they can’t help you.
b.与过去事实相反的假设
条件从句 主句
过去完成时 should（would）等+ have+ 过去分词
例如：If he had come yesterday,I should / would have told him about it.如果他昨天来的话,我会把这件事告诉他的.
含义：He did not come yesterday,so I did not tell him about it.
c.表示对将来不大可能发生的事情的假想
条件从句 主句
一般过去时 should/would等 + 动词原形
were+ 不定式
should+ 动词原形
例如：If you succeeded,everything would be all right.如果你将来成功了,一切都会好的.
If you should succeed,everything would be all right.
If you were to succeed,everything would be all right.
含义：You are not likely to succeed,everything will be what it is now.

1 加到装
2 跟一般现在是
3 跟一般将时

for example:I won't come to the party if it rains!(一般将来时）

虚拟语气~最常见了~主句和从句的时态相差一级~

这里的catch 是名词，具体意思要根据语境来讲，可能是赶上车或者指人也行，
and to think 后面加个句子是个固定用法，后面使用虚拟语气，意思是设想，假想，假如。另外，if引导的从句补全了是if it were not for the salty coffee，表示原因。