A reader’s suggestion, this. Good idea, even though it’s more of a contractual drafting or opinions point than a general legal writing tip.
Shall, will and must
The difference is to a large extent idiomatic – that is, subject more to instinct and feel than hard-and-fast grammatical rules. Usage has also changed over the centuries, and varies between North America and Britain. What follows is a distillation, as best I can manage.
In classic usage, shall is used in the first person (I, we) to indicate a wish or ‘mere futurity’. To say I will or we will indicates an element of intention, volition or choice. So, I shall probably be there but I will never do that.
In North America, I shall and we shall are less commonly heard in the ‘mere futurity’ sense, leaving will to perform both that function and to express stronger intention. This gives rise to ambiguity, because we lose the potential distinction between will and shall. (And shall sometimes performs for us the classic will function, as in We shall overcome.)
In the second person (you) and the third (he, she, it, they), shall takes on the function of command, menace or refusal: You shall repent! or He shall never have any, if I have anything to do with it. Also, Thou shalt not steal, to use the old singular second person (equivalent of tu in French or du in German, as opposed to the plural (or formal singular) vous and Sie.)
In legal drafting, shall is usually used to express a third party’s positive or negative obligations: the Purchaser shall assume all liabilities … Here, shall is synonymous with ‘is obliged to’ or ‘has a duty to’ – command or menace at work. There are arguments that will, if used consistently, would also work: it’s hard to see much of a difference if you said the Purchaser will assume all liabilities – except that shall sounds a bit bossier. But convention (and traditional grammar) are on the side of shall here.
In common parlance must is close to shall in meaning: you must not hit your brother, you shall do this or that. And yet they are not synonymous; in contractual drafting, for example, must often takes on a conditional character: the Borrower must satisfy the following – that is, must do so in order to do something else, but need not.
Should and would
Should is the conditional form of shall. Its use in the first person has all but disappeared from North American usage, but it is classically correct to say I should like to go as a mere expression of desire, and as distinct from the imperative would (I would go, but I can’t).
When used in relation to other people, should is less bossy than shall; it means ‘ought to’, as in you should wash your hands after travelling on the subway. Not dictatorial, if perhaps a bit passive-aggressive.
Should, in opinion-writing, is seen as more qualified than would (this should work as opposed to this would work). William Estey suggests, however, in his book on opinions, that it’s unfair to place the burden on a client to figure out the shades of meaning between the two (and all the other ways we lawyers hedge our own bets). Fair point?
There is also should in this type of construction: Should you wish to leave a message, please do so after the tone. Not incorrect, but fusty – or perhaps fancy for the sake of being fancy. If can always replace should in these instances. Better yet: Please leave a message.
May and might
These can mean the same (or almost the same) when they express possibility: I may go to the party, I might go to the party.
May also expresses permission: ‘You may not go to the party’, the parent told her teenager.
Because of this dual use of may, caution is advised. Avoid using it for negative obligations, since may not could mean either ‘is not permitted to’ or ‘might not’. Better: shall not. In contractual drafting, confine may to the meaning ‘reserves the right to’.
Next time: and/or, and, or.