I disagree with the fairly complex and difficult to remember explanation offered. I fail to really see much difference in using „except“ versus „but“ in this sentence, and I fail to see how using either of them emphasizes anything. I also don’t agree that the placement of either word anywhere in the sentence significantly affects its meaning. I suppose placing „except“ or „nothing“ at beginning of a sentence could place a bit more attention on the word, but that’s a very subtle distinction. It does sound more common to say „he was wearing nothing but his underpants“; however, using „except“ doesn’t really sound wrong to me either, and there’s no major error that comes to mind in hearing it used in this sentence. Every native English speaker would understand „except“ in this sentence and I highly doubt very many would even call it wrong, even if it’s not likely to be most people‘s first choice in a sentence like this.

The word "except" is usually used when that phrase is placed first in a sentence: "Except for the firemen, everyone was now safely out of the building." It is proper to use "but" here... "But for the firemen, everyone was now safely out of the building." ...and this was once common in English. However, it is now very literary and formal sounding. The word "except" can also add weight or focus in a sentence, especially if we use it with unexpected word order, punctuation and/or other key phrases (such as "that is"): "Everyone was now safely out of the building—everyone, that is, except for the firemen." This draws attention to the firemen still in the building. It causes me to worry that something bad may still happen to them. I could have used "but" in that sentence... "Everyone was now safely out of the building—everyone, that is, but the firemen." The unexpected word order, the long dash and the use of "that is" draw attention to the firemen who are still in the building. However, it is not as strong as "except." The word "but" here may cause people to simply wait for the firemen to come out, because it is expected that the firemen would still be inside. The word "except" causes me to go beyond noticing to worrying or expecting something bad may happen. Here is another example where "except" adds more weight than "but": "Anyone else would have died in that fire, but I didn't." "Anyone else would have died in that fire—except I didn't." -or- "Anyone else would have died in that fire, except—I didn't." Notice that I've used the long dash with "except" to add even more focus to "I didn't." The long dash in writing means to take a pause before continuing. What follows may be a mystery that must be solved, or a revelation that this person is actually a vampire.

Both work but the word you need to use is EXCEPT rather than EXPECT. Expect is to anticipate that something will occur. "While he acted surprised he had expected his girl friend to have a surprise party for his birthday." Another example of using except would be, for instance in a setting where you invited someone over for dinner and they asked if they could bring something. "Except for your self we don't expect you to bring anything." Both are natural sounding to me but I would say using except would be used in a more formal type of setting. "Having nothing but the clothes on his back, Henry set off to make his way in the world." "When we got to the market they had nothing but apples and pairs. All the other fruits were already sold." I think the above two sound more natural using "but". "Except for the mission critical staff, everyone else can leave the office two hours early to start the holiday weekend." In this case, as it's a business setting the language would be expected to be a bit more formal so "except" seems more natural to me. Side note. While they don't sound exactly the same except and accept might be words some might confuse when hearing them spoken."We accept all forms of payment except personal checks."

"Nothing but" is a fixed expression that means "only". For example - My cat will eat nothing but the best quality meat =my cat will only eat the best meat. It can be used at the beginning of a sentence - Nothing but the best quality meat for my cat = I will only buy the best quality meat for my cat. In your example, it would be He was only wearing his underpants./He was wearing nothing except his underpants . ( note the difference in structure ) The opposite is "everything but " eg They took everything but /except the kitchen sink on holiday .

"But" is often used in lieu of "Just, Only, simply, merely, nothing but" Here are few examples to make it clear: "I only regret, that I have *but* one life to lose for my country". "He is *but* a shadow of his former self". in above examples "Only" is replaced with "but". :)