Turning a noun into an adjective [duplicate]

a rented house

a confused/bored child

Your mistake is in assuming that "rented" is an adjective here. It isn't; it's a verb. Compare "rented" to "confused" and "bored"

[1] "Rented" can't be modified by "very", but "confused/bored" can: we can say "a very confused/bored child", but not *"a very rented car".

[2] "Rented" can't occur as complement to complex-intransitive verbs like "become": we can say "The child became quite bored/confused", but not *"The house became quite rented".

[3] "Rented" can't occur as complement to complex-transitive verbs like "find": we can say "I found the child quite bored/confused", but not *"I found the house quite rented".

The range of expressions that can occur as pre-head modifier to a noun is very large and varied: we don't want to call them all adjectives. "Confused" and "bored" have the properties of indisputable adjectives and hence must belong in that class, but "rented" doesn't and hence is analysed as a verb phrase in an example like "a rented car".