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Hello,

Could you tell me please what is the difference between 'as...as' and 'not so... as'...?

He doesn't play as well as his sister.

He wasn't so quick a learner as his brother.

He is not so kind as his brother.

He doesn't speak English so well as his brother.

She is not so attentive as her sisiter.

She is not as attentive as her sister.

Once I came across a rule according to which it's a mistake to use either 'not so ... as..', or 'so' and 'as' together one should use 'as as' instead but shoudn't combine so and as either in negative or positive sentences. I wasn't able to find that rule again. But the dictionary says that 'not so... as...' can be used. Was that rule right? I do remember reading it.

Thanks
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Both not as ... as and not so ... as are correct. There's no difference in meaning.

She is not as/so attentive as her sister.

CB
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ninaniaCould you tell me please what is the difference between 'as...as' and 'not so... as'...?
He doesn't play AS WELL AS his sister.
He wasn't SO quick a learner AS his brother.

He is not SO KIND AS his brother.
He doesn't speak English SO WELL AS his brother.
She is not SO ATTENTIVE AS her sisiter.
She is not AS ATTENTIVE AS her sister.
(emphasis added KrisBlueNZ)

In all of those sentences, I would replace SO with AS.
He wasn't AS quick a learner as his brother.

He is not AS kind as his brother.
He doesn't speak English AS well as his brother.
She is not AS attentive as her sisiter.

These are the normal forms. Using SO instead of AS in any of these sentences sounds at least a bit odd, at least to me (I'm a New Zealander, not an American; maybe things are different there).
ninaniaOnce I came across a rule according to which it's a mistake to use either 'not so ... as..', or 'so' and 'as' together one should use 'as as' instead but shoudn't combine so and as either in negative or positive sentences. I wasn't able to find that rule again. But the dictionary says that 'not so... as...' can be used. Was that rule right? I do remember reading it.
That rule isn't complete, I think. You need an example sentence, to see how these words are being used. AS and SO are used in several idiomatic combinations with other words to form commonly used English phrases.

"SO AS TO" - some quotes from Wikipedia:
"Military organization is the structuring of the armed forces of a state SO AS TO offer military capability required by the national ..."
"In economics, the term economic efficiency refers to the use of resources SO AS TO maximize the production of goods and services ..."
As used in these quotes, it seems to mean the same as IN ORDER TO. So you could say "I went to the shop IN ORDER TO get some coffee" (not a very good example, sorry) which means that you went to the shop WITH THE PURPOSE OF getting coffee, i.e. your aim, or intention, was to get coffee, and you went to the shop with that purpose and that intention. That's what IN ORDER TO means.
You can also say something like "You need a driver's license IN ORDER TO legally drive a car". Same idea used differently.
The example "... structuring of the Armed Forces of a state SO AS TO offer military capability ..." means the same as "... structuring of the Armed Forces of a state IN A WAY THAT WILL offer military capability ..." or "... structuring of the Armed Forces of a state WITH THE DELIBERATE (intentional) PURPOSE OF offering military capability ...". So something is done with that specific purpose; with that specific GOAL in mind.

"AS MUCH AS"
"You can take AS MUCH AS you want" = You can take the exact amount that you WANT to take, no matter how MUCH that is.

"AS FAR AS" = "to the extent (or degree or amount) that".
"There's lots of gas in the car, AS FAR AS I KNOW" means what it says - I'm saying I believe there's gas in the car, I know of no reason why there wouldn't be gas in the car, but I'm not absolutely certain that there's gas in the car!
It's just a way of qualifying a statement. Same idea as TO THE BEST OF MY KNOWLEDGE (or RECOLLECTION). Kind of similar to saying "I'm not a lawyer BUT ..." ;-)

"AS MUCH AS I ..."

"NOT SO ... AS TO"
"IN SUCH A WAY AS TO"

Sorry, it's getting late. Google them, alright? :-) I'll elaborate if anyone actually reads this far and asks for more. There are probably a lot more of these phrases.
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Comments  
The only guideline is that 'so/as...as' is used with negative statements and 'as...as' is used with affirmative statements. All your sentences are fine.
 Cool Breeze's reply was promoted to an answer.
 KrisBlueNZ's reply was promoted to an answer.
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