Adding stress to words in a sentence can entirely change the meaning of a sentence. It's incredible, really. So we've got: I'M not going out this evening. (You're not going out, doesn't mean others aren't.) I'm NOT doing anything this evening. (stress on refusing to do anything) I'm not DOING anything this evening. (Someone's implied that you are, perhaps) I'm not doing ANYTHING this evening. (More like a lot of stress on how they'd say it with this one, it could mean an angry tone, annoyed, tired, fed up etc. It'd be an exclamation of the fact that you really aren't going to do something, stress on how) I'm not doing anything THIS evening. (You aren't doing it in that time frame, stress on that day, point of that day.) I'm not doing anything this EVENING. (stress on time) For another example think of this: I didn't say to wear that = someone else said to wear that I DIDN'T say to wear that = denying that I said to wear that I didn't SAY to wear that = I implied it I didn't say to WEAR that = I said throw it/burn it/cut it up I didn't say to wear THAT = You should wear something else

You could also put the stress on "I'm," emphasizing that you're the one not doing anything - but your friend might be. You can put the stress on stress on "anything," which would emphasize that you're refusing to do anything (just like the stress on not). You can put the stress on "this," emphasizing the time. You might even put the stress on "doing," which makes it sound (to me) like you've already said you're not doing anything and now you're repeating yourself. So, really, you can put the stress anywhere in this sentence. :)

Almost any word can be stressed, depending on which element you want to emphasise. For example: A: I can't find anyone to go out with. Everyone's doing something this evening. B: I'M not doing anything this evening. C: Great. Let's go to the cinema.

The first sounds example sounds unnatural to me. I don't think anyone would put a lot of stress into "evening." In the second example, I would probably only say the sentence like this if someone misheard me and thought I was busy or were attempting to insist I go out and do something when I wanted to stay in. I would correct them by stressing that I was NOT doing anything. I think that the word "anything" would naturally have the most emphasis in this sentence. I imagine you would say this sentence because someone would ask, "What are you doing tonight?" You would reply, "I'm not doing ANYTHING this evening." Stress would fall on the object of what you're not doing: anything.