Hard means with great effort or force — "she works hard and leaves late". Hardly means barely or almost not — "she hardly works, she's out by two". The -ly ending does not intensify hard; it reverses it, which is why swapping them turns praise into an accusation.
Hard is already an adverb
Most adjectives need -ly to become adverbs, and writers assume the same must apply here. It does not. Hard functions as both adjective and adverb without changing form:
- Adjective: "a hard question", "the ground was hard".
- Adverb: "he trained hard", "it was raining hard", "she hit the ball hard".
Hardly is a separate adverb of degree meaning scarcely, only just, almost not at all: "I could hardly hear him", "there's hardly any milk left", "we hardly know each other".
The test
Substitute barely. If the sentence keeps its meaning — "I could barely hear him" — then hardly is right. If instead the sense you want is with effort or forcefully, use plain hard. Say the pair aloud to feel the flip: he tried hard versus he hardly tried.
Hardly is a negative — treat it as one
Because hardly already carries negative force, it should not be doubled with another negative. "I can't hardly see" and "there isn't hardly any left" are non-standard in edited English; write "I can hardly see" and "there's hardly any left". This is a real feature of many spoken dialects, so it is a register issue rather than illiteracy, but it is corrected in formal writing.
The same negative quality shows up in two constructions:
- hardly ever — meaning almost never: "we hardly ever go into town".
- hardly … when — for something interrupted almost at once: "I had hardly sat down when the phone rang". Note it pairs with when, not than; than belongs with no sooner.
When hardly opens a sentence, the subject and verb invert: "Hardly had we started when it began to snow." That inversion is formal but standard.
One more sense of hardly
Hardly can also mean "certainly not", used to reject something politely: "That's hardly a fair comparison" means it is not fair at all. Read that way, the word is doing understatement rather than measurement — a common device in British writing.
Note finally the adjective hardy, meaning robust or able to survive difficult conditions ("a hardy perennial"). It is a different word again and is not related to effort.
Frequently Asked Questions
Is it "work hard" or "work hardly"?
"Work hard" — hard is already an adverb meaning with effort. "Work hardly" would mean barely working at all, the opposite of what you intend.
Can I say "can't hardly"?
Not in formal writing. Hardly is already negative, so "can't hardly" doubles the negative. Write "can hardly" instead. It does occur in some spoken dialects.
Is it "hardly had I arrived when" or "than"?
When. Hardly pairs with when: "Hardly had I arrived when the lights went out." Than goes with "no sooner": "No sooner had I arrived than the lights went out."