Common Mistakes : As

 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

 preposition

.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

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

.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

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

(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

.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