@Bochulaz
"She is drying off because she left her panties at home!!"
This line sounds strange because of two reasons:
1. "She is" can be contracted into "She's"
2. The sentence is a statement, which came out of nowhere. Obviously, you can't just change the raws, so you have to make the rest sound like it's a statement before the reader reaches the end of the sentence. This is harder to achieve. I would accomplish this by doing the following:
"She's just drying off because she left her panties at home!!"
"She forgot about the panty?"
This line is incredibly strange for the following reasons:
1. "forgot about the panty" makes it sound like the topic of "the panty" was forgotten about. What is "the panty?" This essentially makes no sense, even with context.
2. The use of the word "panty" is strange, because no one says that. It's like calling a "pair of scissors" a "scissor". "Panties" is a word which is always plural. Similar to "Police". contrast this to how "family" refers to a group (multiple things), but is a singular noun. "Families" is the plural. The plural of "panties" is "panties".
This line should essentially be the following:
"She forgot her panties?"
However there's still an issue with that, it sounds strange because the topic hasn't been brought up yet, as he is thinking to himself. Therefore, he can't springboard off someone else's inner-dialogue. (There are some cases where you do this on purpose, but here it sounds better to do the following):
"Did she forget her panties?"
"W-what are you talking about!"
This line should have a question mark. "W-what are you talking about?!"
"Of course I have not."
This is incredibly stiff. First, it's using the overly formal language with "I have not", which you could argue fits her whole "literature girl" thing, but I'd have to see the raws to be sure of it. Personally, this is much better if you do it as such:
"Of course I didn't."
Side note: I'm not sure what your native language is, but I know that the biggest mistake I see ESL (English Second Language) people make is not using contractions. "Of course I haven't." is still better than "Of course I have not." However, I would still recommend you to do the "didn't" line.
"I didn't put it on."
Simple change to this one, to fix the grammar mistake.
"I didn't put them on."
This is because as stated above, "panties", and "underwear" as well, are plural nouns referring to a singular object. Therefore, you don't put "it" on, you put "them" on. Although, with the previous line using "didn't." I wouldn't use the word "didn't" here again.
Instead I would opt for something which accentuates the joke.
"I never put them on."