As ... as and as

as... as as adverb / preposition

Look at this example:

• He came as quickly as he could.

This structure is used to measure and compare things that are of similar proportion. In this construction, the first as functions as an adverb modifying the following adjective or adverb. The second as functions as a prepositionwhen it relates to the following noun or pronoun. (It can also function as a conjunction when it relates to the following clause.) Compare the following:

• The meal was as good as the conversation: spicy and invigorating!
• She spoke as slowly as she could
• Has everybody eaten as much as they want?
• I hope you will agree that I am as imaginative a cook as my wife (is)!

Note from the above example that if there is an adjective and a noun after the first as, a / an must go between them. Note also that if we want to make a negative statement, we can use so…as instead of as…as:

• He is not so / as intelligent as his sister is.
• The cafeteria was not so / as crowded as it was earlier.

There are a large number of idiomatic expressions or fixed phrases which we use in informal English when we are making comparisons like this. Here are a few of them in context:

• He went as white as a sheet when he saw the ghost.
• My maths teacher is as deaf as a post and should have retired years ago.
• She sat there as quiet as a mouse and wouldn’t say anything.
• Electricity will be restored to our homes as soon as possible.
• All the children were as good as gold when they came to visit me.
• These stories are as old as the hills and have been passed down from generation to generation.

Remember that when we are measuring or comparing things that are of unequal proportion, we need to use the structure comparative + than:

• Let me finish the report. I can type much faster than you (can).
• He played the piece of music more slowly than I had ever heard it played before.

as as subordinating conjunction

Note that as by itself is used as a subordinating conjunction in a variety of different ways.

as = when (for clauses of time)

We may use as as an alternative to when when we are comparing two short actions or events that happened or happen at the same period of time. We often combine it with just:

• She left the house (just) as the sun was rising.
• The telephone rang (just) as I was climbing into my bath.

as = because (for clauses of reason)

We may use as as an alternative to because when the reason is already known or self-evident to the reader of listener. As - clauses are often placed at the beginning of sentences. 

Because puts more emphasis on the reason or introduces new information. Compare the following:

• As Mary was the eldest child, she had to look after her younger brothers and sisters.
• As it had started to rain we had to abandon the picnic.
• I’ve decided to end our relationship because my boyfriend has been cheating on me.

as for clauses of proportion

Here, as means over the same period of time as:

• I think you become more tolerant of other people as you get older.
• As prices rose, the demand for higher salaries became more intense.

as as preposition

Finally, note that as can also be used as a preposition when we want to avoid using the verb to be. Compare the following:

• As his father, it is your duty to ensure that he goes to school every day.
• As you are his father, it is your duty to ensure that he goes to school every day.
• As a social historian, I am always interested in people’s life styles.
• Being a social historian, I am always interested in people’s life styles.
• He established his reputation as a freedom fighter through many heroic acts.
• The police described him as a dangerous criminal.
• The police considered him to be a dangerous criminal