( 1)
What you could have done was to say, "I could have done something in the past but I didn't do it."
What is impossible is equivalent to what was impossible in the past.
(Its negative form only means speculation, not virtuality)
(2) Implicit conditional sentences: In a specific context or at a glance, if conditional sentences can be omitted, or prepositional phrases, adverbs or non-predicate verbs can be used instead of if conditional sentences. This kind of virtual expression is called implicit conditional sentence. In this sentence, otherwise is used as an adverb to imply conditions.
"Otherwise" means if Mary receives my letter.
Supplement: The comma in this sentence should be changed to a period or semicolon, because "otherwise" as an adverb only implies conditions and cannot guide the sentence. If subjunctive mood is guided by if, the complete form is as follows:
Mary can't have received my letter. If Mary had received my letter, she would have replied.
Mary can't have received my letter. If she receives it, she will reply before then.
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I wish you progress in your study!