For and Since
Incorrect: He has been ill since four days.
Correct : He has been ill for four days.
Incorrect: He has been ill for Sunday last.
Correct : He has been ill since Sunday last.
Note: The preposition “for” indicates a duration of time and "since' indicates a past point of time. “For” can be used in any tense. For example:
He worked for two hours.
But "since' is used only in perfect or perfect continuous tenses. ‘For' can be omitted. For example:
How long have you been here?
I have been here two years.
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