Back in the bad old days of New Cambridge English Course and its like, most courses had a completely pointless unit teaching students to say “Turn off the lights” and “Don’t touch the paint”. That is now limited to the Technical English textbooks where it belongs, but that unfortunately leaves other students in need of being told when NOT to use the imperative. That mainly means teaching that in English (unlike Spanish, Japanese, etc, etc), imperative plus “please” is NEVER a request. Such simple don’ts don’t really work, however, so I’ve recently switched to telling them that imperatives are all right when you are offering (“Please go ahead”, “Please take a seat”, etc) but usually should be converted to a request when they are orders/ commands/ instructions (“(Please) drink”, “(Please) sit down”, etc). In fact, it can be useful to get students to divide real life examples of imperatives into commands and offers, probably also converting most of that imperatives into requests.
Here are two worksheets on the topic for realistic proper and probably wrong uses of imperatives:
Other useful things to tell students when covering this point:
– “Please” doesn’t change the meaning of the phrase that you put it with, so “Please shut up” is still a command, “Please make yourself at home” is still an offer, and “Can I have… please?” is still a request
– The differences between a command (“Please don’t smoke in here”) and a request (“Can I smoke in here?”) is that a request can at least theoretically be rejected
– You can use the language of requests for polite commands, but you can’t use the language of commands for requests, so if in doubt use “Could you (possibly)…?” etc
– “Would you…?” is also a command, but “Would you mind…?” can be a request, especially in the form “Would you mind if I…?”
– Request emails end with “Thanks (in advance)/ Cheers” or simply “I’m looking forward to hearing from you (soon)”. Emails with commands/ orders/ instructions can also end with “Thank you for your cooperation”.