Modal verbs, or the full name modular auxiliary verbs, change the meaning of the verb which comes after it. They are the first thing you learn in English (can, could) and also verbs that you keep on learning right up until advanced level. They act in the same way with conditionals and for this first example I'm going to use my favourite football team: Derby County
Using the third conditional:
If Derby County had played better last night, they would have won the match.
Remember: Everything about the third conditional is in the past, and it is totally hypothetical.
So in this sentence Derby County played last night, they didn't play really and they didn't win the match.
Now lets give three similar sentences using different modal verbs.
A) If Derby County had played better last night, they would have won the match.
Cudda, Wudda, Shudda
You've most likely heard this expression and I need to say that the grammar is terrible.
'cudda. wudda, shudda' 'translates' as could have, would have, should have. It's also a little silly to say it.
'Should have' is not really used with conditionals, but it talks about ‘regrets’ and hypothetical situations. It can also be used as a criticism of other peoples action or inaction.
So I'll give you an example: You have a friend talking about a girl he saw last night in a bar, and saying how beautiful she was:
Friend: I was in a bar last night and I saw a girl.
You: Great! What was she like?
Friend: She was so beautiful.