Hey there! I'm happy to help!
11.
Dormir actually isn't a reflexive verb. Dormir is a word that can only be used in regards to oneself, so it is not reflexive as that clarification isn't needed. What do I mean?
Take the word "acostar". This means to put into bed. You could say la mamá acuesta a su hijo (the mom puts her child to bed). However, you could make it reflexive to say Me acuesto (I go to bed).
We have a reflexive form to differentiate between those two things. Dormir doesn't need it because you can't "sleep" someone else so it will never been used in that context.
And with tarde, you just put it at the end of the sentence.
No duermas tarde.
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13. It appears that you are correct. Ponte la ropa is written correctly and sounds correct. No accent is needed in the sentence. It seems like your teacher wants you to write pónte, but that would be redundant and incorrect because in Spanish the penultimate symbol is naturally accented, so an accent is only needed if it breaks that rule or for writing purposes (si vs sí.
Have a wonderful day and keep on learning! :D