(1) I worked hard.

(2) I hardly worked.

(3) I worked hardly.

(4) Hardly, I worked.

(a) The sentences (1) and (3) imply that I worked hard, but sentence (2) says I didn't work at all. Would you please confirm if my understanding is correct?

(b) What is the meaning of sentence (4)?
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rishonly,

Only the first two are used, and they mean very different things (as you have already pointed out) because "hard" and "hardly" have very different meanings in spite of the similarity in spelling.

"hard" means "diligently", "industriously". It tells how you worked. It tells the manner in which you worked.
"hardly" means "almost not at all" (not "not at all"), "little", "not much at all". It tells how much you worked. It tells the degree to which you worked.

CJ
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Thanks, CalifJim.

May I conclude sentences (3) and (4) are grammatically incorrect?. Since an adverb can be used in different positions of a sentence, I thought 'I hardly worked' and 'I worked hardly' have the same meaning.
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The adverb "hardly" is not positioned so freely as other adverbs. It is placed just before the verb. The same is true for its synonyms "scarcely" and "barely". Note that these are adverbs of negative polarity and that "not" itself has the same property. It, too, is placed just before the verb. "almost" -- of positive polarity -- also has this property.

We don't say any of these:

I did work not.
I understood [hardly / scarcely / barely] the explanation.
I won almost the race.

We say:

I did not work.
I [hardly / scarcely / barely] understood the explanation.
I almost won the race.

____________

The elements of negative polarity can also be placed at the beginning of the sentence, but in that case they trigger inversion, just as other negatives do.

"Hardly had I begun to study when the phone rang." (not "Hardly I had begun ...")

CJ
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Thanks, CalifJim. Now I understood the concept.
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