"Close" or "closed"

English as a Second Language English Vocabulary, Grammar and Idioms
1

I saw a sign that read “close on Monday”. My teacher said the sign is wrong. It should read “closed” on Monday. I wonder why close is wrong? Could anyone explain. Thanks

2

Closed on Monday = this shop is not open on Monday.
Close on Monday = this shop is nearby on Monday / you must shut this door on Monday.

If you say “x is close”, the word “close” is used as an adjective or adverb, in which case it means “nearby”. If you want an adjective that means “not open”, you need “closed”.