Past conditionals are used when talking about decisions or actions that you wish had been different. They indicate hypothetical situations from the past using "if" statements.
To form the past conditional, use past perfect in the "if" clause. Verbs in the main clause will depend on whether you are referring to the present or the past. For example*

I wish I hadn't drunk so much last night. Now I feel terrible.
If I hadn't drunk so much last night, I wouldn't feel so terrible now.
(past action and present consequence)
I wish I hadn't drunk so much last night. I felt terrible then.
If I hadn't drunk so much last night, I wouldn't have felt so terrible.
(past action and past consequence)

I wish I had left home sooner. I was late for the party last night.
If I had left home sooner, I wouldn't have been late for the party.
(past action and past consequence)

I wish I had left home sooner. Now I am stuck in traffic.
If I had left home sooner, I wouldn't be stuck in traffic.
(past action and present consequence)

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