As … as
“As + adjective/adverb + as” is used when the things being compared are somehow the same.
Examples:
The temperature today is as low as yesterday. It hasn’t been very warm this past few days.
You have to be as careful as you can in unwrapping this ancient, fragile vase.
She ran as fast as she could to catch up with him.
Not as … as
On the other hand, “not as … as” is used when comparing things that aren’t of equal proportion.
Examples:
The film is not as good as I expected it would be, actually.
The plant hasn’t grown as tall as the others yet.
She’s not as perky as usual today.
Other ways to express “not as … as” are: “not quite … as” or “not nearly … as”.
Examples:
The second exam was not quite as difficult as the first one. (The second exam was difficult but the first one was more difficult.)
This new mobile phone is not nearly as expensive as my old one. (My old mobile phone is a lot more expensive than the new one.)
Although not as common as “not as … as”, “not so … as” can also be used.
Examples:
Cooking all these dishes was tiring but not so hard as the baking we did.
As … as + possibility
“As … as” is normally followed by possibility or ability expressions.
Examples:
Can she submit this document as soon as possible?
Go to the gym as many times as you can a week.
They got to the hospital as fast as they could after hearing about his accident.
As much as / As many as
In terms of making comparisons about quantity, “as much … as” and “as many … as” are used with uncountable and plural nouns, respectively.
Examples:
Sean makes as much money as David but not as much as Dion.
Mia’s parents try to give her and her brother as much freedom as they can.
There weren’t as many tourists on the beach as we expected last weekend.
“As much as” and “as many as” followed by a number are used to refer to a great number of something.
Examples:
There are as many as 8.6 million people living in New York.
The United States produces as much as 230 million tons of garbage per year.