There is nothing inappropriate (that anyone can see here) in the way that you've written your email. The response from the professor suggests that they're some combination of (a) incredibly time-constrained, (b) sloppy and unclear in how they communicate, and (c) a jerk.

• It's possible (as user2768 suggests) that the essential "offense" in their eyes was to ask about things available in the online site. If so, then their first sentence was a sloppy and mostly mindless piece of aggression that doesn't really mean what it says.

• The curtness, snarkiness, and level of aggression communicates that they don't like interacting with students by email, do not want to spend time on it, and want such querents to go away and not bother them.

• Given the above, writing them again and asking for more time to be spent on even smaller minutiae -- like revisiting what they meant in a hastily-written throwaway email, and to spend even more time parsing the grammar of the original student email that they've already expressed contempt for handling -- is only likely to make things worse.

I suggest that you accept this line of inquiry as not useful and likely to produce only further confusion and frustration on both sides. Do not email them again on this matter. If you must email them again in the future, on some different matter, make it as brief as you possibly can (shorter than your original email), and try to pose a question that can be given a very short, clear-cut answer.

I'll say that the only thing the professor really got right there was in accurately predicting your next instinct would be that, "very soon you will be writing many more official emails", and to be aggravated-in-advance by that prognostication. Resist the temptation to fully prove them right!

Also, consider asking around if this professor is good to interact with (maybe people you know or a ratings website). If their course is not required, then you may be getting a signal that they're just not a good professor.

Your language isn't the problem, your email is well-written, but you've seemingly wasted the professor's time. You could have looked up the information, as they have explained: You may see course content on the department webpage.

Comments suggest I'm ignoring the professor's words:

you must know how to address them appropriately

The word address can mean speak to, perhaps write, in this instance. I believe the professor is annoyed that you've written to them and you should have done your own research first. An appropriately written email would ask a specific question beyond what's online.

Given you've seemingly annoyed the professor, I suggest you don't respond.

Your email is impeccably written. If I compare it to the emails I receive from my students, it would fall in the 99th percentile in terms of email etiquette, grammar, formatting, and including the information relevant to the question you are asking.

Your email also compares very favorably with the professor’s reply, which violates several standard rules of email etiquette. Seems to be a case of the pot calling the kettle black if you ask my opinion.

It appears that the professor's response was somewhat abrupt. To address this situation, let's break down what might have gone wrong and how you can improve your future emails. (i) How to respond to this: Given the tone of the professor's response, it's essential to maintain a professional demeanor. You could write a polite yet concise reply, acknowledging their feedback and expressing appreciation for their time. Here's a possible response: Dear Prof. Last Name , Thank you for your time and for pointing out my mistake. I will make sure to address professors more formally in my future emails. I have checked the department webpage, but I would appreciate it if you could confirm the course content and prerequisites. Looking forward to hearing from you. Best regards, Name Department Institution (ii) What went wrong: It seems that the professor was taken aback by your informal greeting and closing. In academic settings, it's generally more appropriate to use formal titles (e.g., "Prof." or "Professor") followed by the last name, and a more formal closing (e.g., "Sincerely" or "Best regards"). Additionally, checking the department webpage for course content is a good idea, but it's often helpful to verify this information with the professor, especially if you have specific questions or concerns. In your email, you asked about the course content and prerequisites, but it's possible that the professor assumed you had already looked at the webpage and were looking for further clarification. To avoid this situation in the future, make sure to: * Use formal titles and last names in your greetings and closings. * Check the department webpage for general information, but also confirm with the professor when needed. * Be clear and concise in your questions, and avoid assuming the professor is familiar with your previous courses or experience. By following these guidelines, you can maintain a professional tone and build rapport with your professors, even if your initial email doesn't quite hit the mark.