Accustomed ____ , he was surprised he had fallen so far behind with the deadline.
a. to working hard
b. to work hardly

Can you explain what is the difference in the meaning between (a) and (b)? (pls)
a is closest, but neither is correct. It should be:
Accustomed though he was to working hard, he was surprised...

The part before the comma needs to be be a complete clause.

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Because ' to ' is a preposition, it should be followed by a ' noun ' or ' gerund '. So ' to working hard ' is the choiceEmotion: smile
Why does the part before the comma need to be a complete clause?
It doesn't, it needs to be an adverb, or a phrase which acts as an adverb. "Working hard" isn't a clause, nor is "Accustomed to working hard", nor is "Accustomed though he was to working hard". All of these phrases parse as adverbs, because the verb, "working is a participle (it ends in -ing), which changes it - together with its subject and object if they are supplied - into an adjective or an adverb; in this case, an adverb.

I don't think I explained that very well. Maybe someone else can add something. If it's still not clear, ask more.

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