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could anybody please tell me if this sentence is correct :
I could not be able to attend school on Monday
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No, it's not.
Use one of these:

I was not able to attend school on Monday. (past)
I will not be able to attend school on Monday. (future)
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Hello, hmmm—and welcome to English Forums.

'Can' = 'be able to', so your sentence is wrong. Perhaps you mean one of these:

I could not / was not able to attend school (last) Monday.
I cannot / am not able to attend school (next) Monday.
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Comments  
oh .. thank u:)
 Mister Micawber's reply was promoted to an answer.
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oh ok ... thank you so much Emotion: smile