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Past subjunctive. Expresses unreal actions.
In simple sentences. Denotes wish or regret (If only he had come!; Oh, that the rain were over!), preference or advice (We had better get down to work; I would rather study a bit). Can be used with modal verbs. It is followed by an infinitive (Where could we stay for a night?). The non-perfect form is homonymous with the Past Simple, the perfect form is homonymous with the Past Perfect and represents an action which is referred to the past and is contrary to the reality.
In complex sentences. In Nominal Clauses it can be used in Attributive Clauses introduced by clauses that begin with "it's (about, high) time" (non-perfect only) (It's about time you stopped talking). In Predicative Clauses it is used after link verbs "to be, to look, to smell, to sound, to feel, to seem, etc." + "as if, as though" (You look as though you had never seen it before). In Object Clauses it's introduced by the verb "to wish" and the modal expression "would rather" (I wish you were here; I'd rather you called her). Used in the indirect questions after the expression of doubt (I doubt that it were possible).
In Adverbial Clauses it is used in clauses of manner and comparison after the conjunctions "as if, as though" (He spoke as if he were going to cry). It is also used in adverbial clauses of condition (I shouldn't take it if I were you) and in adverbial clauses concession (after "even though") (Even if they had wanted to release the album, it would have been a failure).