How many different forms of adjectives are there and where are they placed?

There are (1) one single word (一個字)

For example: big, small, happy, sad, outstanding........      

It is placed before the noun, after the noun and after the verb it completes

For example: He is a happy child.

He wrote a poem short and beautiful.

She looks happy.         

(2) a phrase (一個片語)

For example: Reading carefully, Mary found what she needed in the book. (現在分詞片語)

Arrested by the police, David admitted what he had done. (過去分詞片語)

The pencil on the table is mine.(介詞片語)

The way to proceed has not been discussed. (不定詞片語)

It is placed immediately before or after the noun it modify.

(3) a clause (一個子句)

For example: The boy (who is) standing by the tree is David. (關係代名詞子句)

 I cannot find the book which (or that) I bought last week . (關係代名詞子句)

It is placed immediately after the noun it modify.