Modal verbs are small words with large social consequences. "You must send it" and "Could you send it?" request the same action and produce very different reactions.

Ability: can / could

  • "I can speak three languages." (present ability)
  • "I could swim when I was six." (past ability)

Requests, from direct to polite

  1. "Can you send it?" — direct, fine with colleagues
  2. "Could you send it?" — softer, safe almost anywhere
  3. "Would you mind sending it?" — most polite

Note that "Would you mind…" is answered confusingly: "No" means yes, they will do it.

Advice: should

  • "You should check with HR." (a recommendation)
  • "You shouldn't reply yet." (advice against)

Obligation: must / have to

  • "I must finish this tonight." (internal — I have decided)
  • "I have to finish this tonight." (external — someone requires it)

Careful: "mustn't" means it is forbidden, while "don't have to" means it is optional. These are opposites, and mixing them causes real confusion.

Certainty

  • "He must be stuck in traffic." (I am fairly sure)
  • "He might be stuck in traffic." (possible)
  • "He can't be stuck — I just saw him downstairs." (impossible)

The grammar rule that never changes

Modals are followed by the base verb, with no to and no -s:

  • "She can go." Not "can to go", not "can goes".
When in doubt in a professional setting, "could" is almost never wrong.