I
conjunction
BAD : As our room was upstairs, so we didn't hear him.
GOOD : As our room was upstairs, we didn't hear him.
GOOD : Our room was upstairs and so we didn't hear him.
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See note at SO 2 (↑ so )
II
preposition
1
BAD : His skin is not as the skin of a young man.
GOOD : His skin is not like the skin of a young man.
BAD : It looked very fragile and so I handled it as china.
GOOD : It looked very fragile and so I handled it like china.
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In comparisons, the word that is usually used in front of a noun or noun phrase is like (NOT as ): 'James is very tall, just like his father.' 'Their car is like ours - old and full of rust.'
As is used in the patterns (not) as ... as, not so ... as, and the same (...) as : 'James is as tall as his father.' 'Their car is the same colour as ours.'
2
BAD : I was treated as if an old friend.
GOOD : I was treated as if I was/were an old friend.
GOOD : I was treated like an old friend.
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See also WAS (↑ was )
3
BAD : The book is concerned with important social issues as child abuse and women's rights.
GOOD : The book is concerned with important social issues such as child abuse and women's rights.
GOOD : The book is concerned with important social issues like child abuse and women's rights.
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Examples of a class or category are introduced by such as or like (NOT as ): 'Serious diseases such as AIDS and cancer can cause a great deal of suffering.'
4
BAD : She looked at everyone as she were their superior.
GOOD : She looked at everyone as if she were their superior.
BAD : I suddenly felt as I was fifteen again.
GOOD : I suddenly felt as if I was/were fifteen again.
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See also WAS (↑ was )