If you tried to force
“proper written sentence” to stand in for
“properly written sentence,” it would be
grammatically off, because:
- written is a past participle functioning like an adjective,
- so if you want to describe how it was written, you need an adverb (properly), not an adjective (proper).
That’s why
“a proper written sentence” (no comma) sounds a little “off” in standard English — it’s like you’re treating
written as just another descriptive adjective instead of something that needs an adverb to qualify it.
The only way to make
“proper written sentence” grammatical is the
“proper, written sentence” structure you mentioned, where
proper and
written are clearly two separate adjectives. But in that case, it’s no longer interchangeable with
“properly written sentence.”

So your instinct is right:
- properly written sentence = correct way to say it when talking about writing quality.
- proper, written sentence = a different meaning altogether (two adjectives stacked).
- proper written sentence (without comma) to mean “properly written” = grammatically wrong.
Want me to show you how linguists/grammarians would diagram “properly written sentence” vs “proper written sentence” to make the rule crystal clear?