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as と if の間に「主語+助動詞の過去形+動詞の原形」が省略されているからです。
This sentence looks as if it were the subjunctive mood.
This sentence looks as [ it would be written ]if it were the subjunctive mood.
If this sentence were the subjunctive mood, it would be written as such.
as if 節の表す内容が、主節の動詞が表す時と同じ時に起こったことなら、as if 節の動詞は常に過去形、主節の動詞より古いことなら常に過去完了形。このようなas if 節の動詞の変化を仮定法(the subjunctive mood)と呼びます。