The difference between Decent and Proper

When used as adjectives, decent means appropriate, whereas proper means suited or acceptable to the purpose or circumstances.

Proper is also adverb with the meaning: properly.

check bellow for the other definitions of Decent and Proper

Decent as an adjective (obsolete):

Appropriate; suitable for the circumstances.
Decent as an adjective (of a person):

Having a suitable conformity to basic moral standards; showing integrity, fairness, or other characteristics associated with moral uprightness.
Decent as an adjective (informal):

Sufficiently clothed or dressed to be seen.

Examples:

"Are you decent? May I come in?"
Decent as an adjective:

Fair; good enough; okay.

Examples:

"He's a decent saxophonist, but probably not good enough to make a career of it."
Decent as an adjective:

Significant; substantial.

Examples:

"There are a decent number of references out there, if you can find them."
Decent as an adjective (obsolete):

Comely; shapely; well-formed.

Examples:

"A sable stole of cyprus lawn / Over thy decent shoulders drawn"

Proper as an adjective:

Suitable. Suited or acceptable to the purpose or circumstances; fit, suitable. Following the established standards of behavior or manners; correct or decorous.

Examples:

"the proper time to plant potatoes"

"a very proper young lady"
Proper as an adjective:

Possessed, related. Used to designate a particular person, place, or thing. Proper nouns are usually written with an initial capital letter. Pertaining exclusively to a specific thing or person; particular. In the strict sense; within the strict definition or core (of a specified place, taxonomic order, idea, etc). Belonging to oneself or itself; own. Portrayed in natural or usual coloration, as opposed to conventional tinctures. Being strictly part of some other (not necessarily explicitly mentioned, but of definitional importance) thing, and not being the thing itself. Eigen-; designating a function or value which is an eigenfunction or eigenvalue.

Examples:

"usex [[proper subset]] — [[proper ideal]]"
Proper as an adjective:

Accurate, strictly applied. Excellent, of high quality; such as the specific person or thing should ideally be. (Now often merged with later senses.) Attractive, elegant. In the very strictest sense of the word. Utter, complete.

Examples:

"Now that was a proper breakfast."

"When I realized I was wearing my shirt inside out, I felt a proper fool."

Proper as an adverb (UK, colloquial):

properly; thoroughly; completely
Proper as an adverb (nonstandard, colloquial):

properly