Write a new email or reply to one: the right tone, every time.
Most student emails fail on structure, not politeness. A lecturer reading fifty messages needs to know within one line who you are, which course you mean, and what you are asking for. Warmth is welcome; ambiguity is what gets an email left for later.
Course code and the request, not "Question". A subject line is how a message gets found again in a full inbox.
Who you are, which class, and what you need. Context comes after, because context first buries the request.
Formal enough to respect their time without being stiff. Deference is not the same as clarity, and an over-apologetic email is harder to answer.
It uses what you provide. It does not invent a reason, a date or a circumstance on your behalf.
Ask early, say plainly what you need and by when, give the reason in one sentence without over-explaining, and say what you have already done on the work. A specific new date is easier to grant than an open request.
Both are fine at most institutions. "Dear Dr Surname" is the safe default for a first email; follow their lead afterwards, and use the title they use for themselves.
Short. Under 150 words for most requests. If it needs more, the extra usually belongs in an attachment or an office-hours conversation.
You can try it for free. A free account saves your drafts across devices.