English Tense 問題一問?

I have been receiving letters
=The present perfect continuous tense
=began in the past; has extented up to the present;may continue in the future,
Hence:-I have been receiving letters up until now(=no letters.)
I have received letters
=present perfect tense;
=present results of past actions
=ever, never, for and since are used in Usage.
I have just received my letter.-----just-----the immediate past
I have already received the letter.-----already----indicates completion
The sequence of tense should be used:-If the verb in the principal clause is in the past tense, the verbs in the subordinate clauses should also be in the past tense.
However, in the case of noun clauses, the simple present tense is allowable if it denotes a general truth.
If there is a present or future tense in the main clause, any tense may bre used in the subordinate clause depending on the need.
eg:-I know what I received.
eg:-I shall go when the letter burnt.--special uses with adverbial particles prep,away.)
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