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AFAIK the correct grammar for "speak slow" is "speak slowly" (slowly being an adverb). Please correct me if I am mistaken.

But in daily life I have not heard anyone saying "Speak slowlier".

I think I heard folks saying "Speak slower" but I also think it is incorrect.

Which is the best or correct form to convey the message?

Could that be the longish "Speak more slowly?"

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@MattЭллен Your linked question does not point out that comparative degrees of -ly adverbs do not become *-lier ones. – tchrist Aug 14 '12 at 13:58
@MattЭллен Your link is great but AFAICT it focuses on adjectives. Adjectives are easy. :) – srf Aug 14 '12 at 14:07
OK, It's not a duplicate. – Matt E. Эллен Aug 14 '12 at 14:11
replace slow with loud and think again – Shiplu Aug 14 '12 at 16:10
up vote 16 down vote accepted

Slow can be an adverb as well as an adjective. Not all native speakers know this, and some will tell you it's wrong to say 'Speak slower'. If you don't want to risk upsetting them, you can certainly say 'Speak more slowly'.

The Oxford English Dictionary has two citations for slowlier, including this from the seventeenth century philosopher Thomas Hobbes: 'They marched the slowlier for the rain which had fallen the same night.' It is, however, rare: I have never seen or heard it until now.

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Your answer seems to be the most authoritative thus far (it actually brings citations). Accepting. – srf Aug 14 '12 at 14:12
@srf:Thank you. – Barrie England Aug 14 '12 at 14:16
+1, but saying slow can be an adverb as well as an adjective might be somewhat confusing, if it gives the impression that you can use slow anywhere you can use slowly. You can't. I don't know the grammatical rules for when you can use flat adverbs (like slow), and I believe they're different in the U.S. and the U.K. – Peter Shor Aug 14 '12 at 14:49
@Peter Shor: I don't know either, but it may be that when 'slow' is an adverb it more frequently comes after the verb rather than before it, whereas 'slowly' can be in either position. – Barrie England Aug 14 '12 at 15:19
+1 - and as a native English speaker, I've also never heard "slowlier" before. – Izkata Aug 14 '12 at 18:02

Slower can either be an adjective, or an adverb.

"Speak slower" is grammatically correct. "*Slowlier" is not a word.

"*Slowlier" sounds very unnatural to a native English speaker; however, not all know that slower can also be an adverb.

Some incorrectly claim that the adverb "slower" should be avoided. However, it has been in use for about four centuries now and is well established as a real word.

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Thanks +1 but please note @BarrieEngland's citations: Slowlier may be arcane but it does seem to be a word. – srf Aug 14 '12 at 14:11
Albeit extremely uncommon. I have never heard it in spoken or written speech, and Barrie himself says the OED has only two citations for it, which are archaic. If it was ever used, it is archaic now. If you use it, be prepared to be corrected, rightly so. – American Luke Aug 14 '12 at 14:13
I don't suggest using it, but you have to be very careful in saying that something isn't a word. If you do, it's probably advisable to say what you mean by 'word'. – Barrie England Aug 14 '12 at 14:22
@Luke: That's one way of defining 'word', but it's not the only way. The textbook definition, which is not itself without difficuties, is something like 'a unit of grammar that is more than a morpheme but less than a phrase'. – Barrie England Aug 14 '12 at 15:16
I think "If you use it, be prepared to be corrected" is a pretty good practical definition. – tenfour Aug 14 '12 at 17:10

I am a native English speaker and I only use "slowlier". "Slower" would be incorrect and "more slowly" would be acceptable, but awkward.

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I'm sorry, but this answer is just totally incorrect. – Marthaª Mar 20 '14 at 14:59

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