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Which is incorrect, IYO?

I should go to church, but I'm not going to.

I must go to church, but I'm not going to.
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I'd say the latter is incorrect.
Even after a long lunch, on a Friday afternoon, when you sit down at your desk and say to yourself, "I must finish that report. But I'm not going to..."?

MrP
Comments  
I would use should.

Should is the right thing to do. Must is more a necessity than a choice.

Ikia.
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The semantic of the second one may sound a bit rough by natural English standard.

The first one is better understood and perhaps accepted than the latter,
GoodmanThe semantic of the second one may sound a bit rough by natural English standard.

The first one is better understood and perhaps accepted than the latter,

Thanks. I'd say the latter is incorrect.
 MrPedantic's reply was promoted to an answer.
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MrPedantic
I'd say the latter is incorrect.
Even after a long lunch, on a Friday afternoon, when you sit down at your desk and say to yourself, "I must finish that report. But I'm not going to..."?

MrP

Only if "I'm not going to" means "it's impossible to do so due to external circumstances that will prevent me doing so".

Or if the "I must go to church" means "my mother says I must go" (prohibition). If the sentence means what I meant it to, I would say it is incorrect.