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You will be in trouble if you do it again.

You will be in a big trouble if you do it again.

Is ''a'' necessary'' for the 2nd sentence? (Please explain)

Same thing:

I took her out for dinner.

I took her out for a romantic dinner.

Is ''a'' necessary for the 2nd sentence? ( Please explain)

He is fat compared to Sam.

He is fat when compared to Sam.

He is fat when you compare him to Sam.

Which of these is correct?
Comments  
You will be in trouble if you do it again - general trouble, not just one specific difficult situation.

You will be in a big trouble if you do it again- "a" accentuates the individuality of that "trouble", its uniqueness.

Is ''a'' necessary'' for the 2nd sentence? (Please explain)

Same thing:

I took her out for dinner.- ordinary situation, casual outing.

I took her out for a romantic dinner.- not just any casual dinner, but a romantic one.

Is ''a'' necessary for the 2nd sentence? ( Please explain)

He is fat compared to Sam.- ok

He is fat when compared to Sam.- ok ( passive voice, less used in this context)

He is fat when you compare him to Sam.- 0k also.

Which of these is correct?
Thanks for the reply.

So is ''a'' necessary if it is a stand-alone sentence?
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