RULE#4 => WISH + COULD = regret in the present concerning lack of ability.
|RULE#1||WISH (IF ONLY) + PAST SIMPLE||This pattern is used to express regrets or wishes about the present (or future).
I wish I had a car.
|RULE#2||WISH (IF ONLY) + PAST CONTINUOUS||It is used to express that we want to be doing a different action in the present (or future).
I wish they weren't leaving next week.
|RULE#3||WISH (IF ONLY) + PAST PERFECT||We use it to show we regret something (we want something in the past to be different).
We wish we had taken his advice.
|RULE#4||WISH (IF ONLY) + COULD + INFINITIVE||Regret/wish in the present concerning lack of ability.
She wishes she could speak French.
|RULE#5||WISH (IF ONLY) + WOULD + INFINITIVE||It is used to express annoyance, impatience or dissatisfaction with a present action.
I wish you would stop drinking.
Wish and would must have a different subject!