This is a brief primer on writing an email to your professor. I love hearing from students, and I love helping my students succeed. I also love email, which is at least in theory an ownerless, anarchistic, open protocol that doesn't rely on any one company or entity to make it work. But often I find that students come to college in need of a few pointers on what an effective email looks like.
Be cordial. It's important to signal respect when asking for someone's time and mental energy. Address your email to “Professor ” (unless you have previously addressed each other more informally), and sign off with your name at the bottom. Emails aren't formal letters, but if they're too conversational, or include bad grammar or punctuation, they seem lazy, which will encourage laziness on the part of the recipient.
Be brief. Too many people spend too much of their lives weeding through email. For a professor, every email is another few minutes they're not spending on their research or mentorship. Help make the world a better place for everyone by keeping your message to the point and saving the details for an in-person conversation.
Do your homework. Before asking a question about the assignments, expectations, or other class-related issues, be sure you have read the syllabus and any related correspondence from the professor. Better yet, reference those materials when you ask your question, explaining what about them isn't clear. This demonstrates that you are serious about taking responsibility for your work and that your question is in good faith.
Be precise. Describe with as much directness and precision as you can what you're having trouble with. Don't ask for general, broad information about the course or assignments—save that for an in-person discussion. Email is great for quick questions with quick answers.
Respect the recipient's time. We all have different kinds of demands on our time, and try to respect that. Avoid statements like “respond as soon as you see this” and “URGENT!” If you write outside normal working hours, in particular, do not expect an immediate response. In general, assume that any email may not get a response for a few business days.
Share your passion. Professors are professors because they're passionate about what they do. We're much more likely to want to help you if we notice that you're passionate too. If appropriate, mention something about the course or your question that is exciting your enthusiasm.