The difference between hard / hardly

Question

I had a discussion with a colleague at work.

1. I was not able to explain why the things are the way they are. I explained him the meanings and so on, with the reason "because". Topped with "take it or leave it". However, I want to be more professional than that.

2. Also, I worry that I do not properly understand the difference between "adjective" and "adverb" in English.

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1. This is hard work.

"Hard" is adjective (sits with "work" - a noun).

2. I work hard.

"Hard" looks like an adjective (see above) but it works like an adverb (sits with "work" - verb).

Also, we can transform adjectives to adverbs using the suffix "-ly":

bad -> badly

However, the following is definitely incorrect:

3. I work hardly.

While the following is correct, but it means pretty much the opposite of 2:

4. I hardly (=barely) work.

Answer 1

English is Germanic.

Therefore we have unmodified adjectives functioning as adverbs.

This usage doesn’t have unlimited application though.

Generally the best adjectives are short (in terms of syllable count) and primitive (rather than borrowed or derived).

The consequence is that some adjectives sound wrong when used in certain roles.

It might be that the mere availability of a possible alternative (“hard”) makes interpretation of the explicit adverb more difficult than otherwise. If so “hardly” is wrong because there are two opposing meanings to the adverb and one other meaning to the adjective—and the alternatives don’t have the same explanatory functions in clarifying the meaning of the sentence.

Clearly two non-syntactically equivalent items cannot be used in tandem with a single conjunction (for instance).

The attempt to parse your “hardly” statement may just end without any clear candidate representing the intended meaning of the sentence.

The computer works—hardly. (Minimally functional.)

*The computer works electronic. (Long, non-primitive adjective fails as adverb.)

I work hard. I run hard. (Intensity. Short, primitive adjective succeeds as adverb.)

I work—hardly. (Limitation of degree.)

I hardly work. (Limitation of degree.)

*I work hardly.
Failure Steps: 1. Check superficial plain meaning: limitation of degree? Possible. 2. Contrast to alternative meanings/constructs: Is operational? Reduced applicability to personal agent/subject. Exerts itself? Possible. In a hard manner? (I.e., as the adverb denoting the manner corresponding to the adjective.) Possible—unclear.

3. Resolution of meaning No direct comparison possible. Failure.

4. Conclusion: ungrammatical.

Answer 2

The original meaning of hardly was "in a hard manner" but it transformed into basically the opposite of that in Modern English. There's a good answer here by RegDwigнt on English StackExchange that links to the word's etymology on Etymonline:

hardly (adv.)

c. 1200, "in a hard manner, with great exertion or effort," from Old English heardlice "sternly, severely, harshly; bravely; excessively" (see hard (adj.) + -ly (2)). Hence "assuredly, certainly" (early 14c.). Main modern sense of "barely, just" (1540s) reverses this, via the intermediate meaning "not easily, with trouble" (early 15c.). Formerly with superficial negative (not hardly). Similar formation in Old Saxon hardliko, German härtlich, Old Danish haardelig.