In English, adjectives come before substantive nouns, e.g. "Red dress".

red: adjedctive - dress: substantive noun.

Why do we put adjective after "something" although that "missing" is adjective and "something" is substantive noun.

- This is somthing missing in this room. NOT "This is missing something in this room."

Thanks & Regards

I think it's idiomatic. We say it that way because that's how we say it.

Michael Swan discusses this in his Practical English Usage, Section 16 on Position of Adjectives. He notes that adjectives also follow other words like everything, anything, nothing, somebody, anywhere.

In spoken and informal English, you'll sometimes hear the adjective placed in front, eg He bought her a special something for her birthday.

Best wishes, Clive
Thanks for your reply