Why "我想了退这件新买的毛衣。” is wrong? The correct sentence is &quot;我想退了这件新买的毛衣。&quot;  But why?
Please do not use grammar jargon because I seldom understand them.
I have been teaching Mandarin with grammar included but never mention and grammar term in my teaching and even the word &quot;grammar&quot; itself, simply because I am a victim of (English) grammar: hours spent just to memorize those stupid names, mind you, both in English and Chinese and their function/usage.
Instead, I use &quot;logic&quot; to teach grammar because Chinese is one of the most logical and cultural language in the world.
我想退了这件新买的毛衣。

Semantically, 我想 itself connotes a completion already because when you say it you already think about it.  So we don't need 了 after 想 for completion.  That's why the sentence in title is not correct.
退了 denotes I want the action 退 get done. &quot;Done&quot; is for completion, so we need a 了 after 退.
P. S.  I somehow agree with you Chinese language is easier to learn if you focus more on logic, convention and culture, especially for second language learners.  Chinese grammar is the hardest part of the language. Only few people truly master Chinese grammar. Second language learners can easily create a wrong/stilted sentence based on the grammar rules they learned.