subjunctive_conditional
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In the sentence "I wish you were here," the past subjunctive tense is used. The verb "were" is the past subjunctive form of the verb "to be." The past subjunctive is used to express hypothetical or unreal situations, desires, or wishes.
In the sentence "I wish I could fly," the past conditional tense is used. The verb "could" is the past conditional form of the verb "can." The past conditional is used to express hypothetical or unreal conditions or abilities in the past.