as ... as
When you are comparing one person or thing to another, you can use as followed by an adjective or adverb followed by another as.
The ponds were as big as tennis courts.
I can't run as fast as you can.
After these expressions, you can use either a noun phrase and a verb, or a noun phrase on its own.
François understood the difficulties as well as Mark did.
I can't remember it as well as you.
If you use a personal pronoun on its own, it must be an object pronoun such as me or him. However, if the personal pronoun is followed by a verb, you must use a subject pronoun such as I or he.
He looked about as old as me.
You're as old as I am.
You can put words and expressions such as almost, just, and at least in front of as ... as structures.
I could see almost as well at night as I could in sunlight.
He is just as strong as his brother.
used with negatives
You can use as ... as structures in negative sentences.
They aren't as clever as they seem to be.
I don't notice things as well as I used to.
You've never been as late as this before.
So is sometimes used instead of the first as, but this use is not common.
Strikers are not so important as a good defence.
used for describing size or extent
You can use expressions such as twice, three times, or half in front of as ... as structures. You do this when you are indicating the size or extent of something by comparing it to something else.
The volcano is twice as high as Everest.
Water is eight hundred times as dense as air.
using just one ‘as’
If it is quite clear what you are comparing someone or something to, you can omit the second as and the following noun phrase or clause.
A megaphone would be as good.
This fish is twice as big.