English language

Conditions

 
The general rule:
The sentence with condition is usually as follow:
Main clause       IF  / UNLESS         subordinate clause  
(Unless = Not if)

 
1. For something that we can realize, we use:
Future    IF/UNLESS   Present
Examples:
I'll come if you invite me.
I won't come unless you invite me.

2.  For something that we can’t realize, we use:

2.1. In the present:
Would + verb IF/UNLESS simple past
Examples:
I would buy a car if had enough money.
I wouldn't buy a car unless I had enough money.
2.2. In the past:
Would + have + past participle IF/UNLESS past perfect
Examples:
I'd have come if you had invited me.
I wouldn’t come unless you had invited me.
Had he came yesterday, he wouldn't have been late.