You might be thinking that the adjective "usual" in English means exactly the same thing as the adjective "usual" in Spanish, but they're falsos amigos. While your constructions are correct in a technical sense, you shouldn't use the adjective "usual" in this way. It's more natural to use the adverb "usually" or "normally": - People usually prefer to sleep at night. - People normally go to work in the morning. You can use the adjective "common" to express the same idea: - It is common for people to sleep at night. - It is common for people to go to work in the morning. To use "usual" as an adjective in a natural way: - David had his usual breakfast of bacon and eggs, then took his usual train to work. The second construction is also correct in a technical sense, but should be avoided because it's very confusing. I think you're trying to say "it's nearly impossible for young people to have time for exercise" but using the double negative reverses the meaning, so your example literally means "it is possible that young people have time for exercise." In both cases, it sounds like you're trying to translate a Spanish construction into English word-for-word ;-)

1. It is usual that many people prefer.... 2. It is usual for people to prefer... Combining "It is usual" and "for" is more natural sounding (to me, American) than combining with "that." So for #1 I would say "It's usual for most people to prefer..." 3. Does it work for most adjectives? I wouldn't say yes, but there are a lot that do work. But some don't sound natural with "for" and other don't sound natural with "that" and still others sound fine with either. Ie. It's usual/common/possible for... It's interesting/concerning/frightening/funny/possible that... (This has many, many options.) 4. It is hardly impossible that young people have time to do exercise/ for young people to do ex. This has a meaning of "Young people should have time to exercise." It sounds more natural to say "It's hardly possible that young people don't have time to exercise." This spreads the double negative ("hardly impossible") and removed the "do" before exercise, which is already implied as a "doing" activity. Hope this helps!