This is my OPINION: 1. The complete sentence should be "When would IT be convenient for you to come?" a. Native speakers do often leave out the "it" in fast speech. 2. In order to parse (analyze) your sentence, however, we need the COMPLETE sentence. 3. When you have a question, and you want to parse it, you must return it to "regular" order. 3. Thus, we get: "It would be convenient when for you to come? a. As you can now see, the formal (grammatical) subject is "it." That "it" means nothing. It is only a substitute for the "real" subject, which is "for you to come." b. But native speakers do not like to say "For you to come would be convenient when?" Tom: When would it be convenient? Mona: I do not understand you. Convenient for what? Tom: For you to come. Mona: Oh, now I understand. It would be convenient for me to come on Saturday. ***** It = pronoun. would be = linking verb. convenient = adjective (refers to "it"). when = interrogative adverb that modifies the verb "would be." for you to come = prepositional phrase (the "real" subject).

"When" is the subject. You're asking a question, remember.