Must I? Should you? They might.
Modal verbs are used to modify the strength or mood of other verbs. There are many modal verbs (can, will, could, would, should, shall, must, may, might), but the two most problematic modal verbs are "should" and "may".
"Should" has three meanings (encouragement, recommendation or expectation) and is often used (incorrectly) when the writer's intention should [expectation] be "must". Writers often use "should" when "must" is required over a misplaced fear of causing offence. Trust me, you aren't doing the reader any favours by trading politeness with ambiguity.
"May" also has three meanings (ability, possibility or permission) and is another common cause of ambiguity. If used for anything other than permission, "may" should [recommendation] be replaced by "can" or "might", depending on the intended meaning (see below). My personal choice is to never use "may" in a technical document because there is always a better word available to describe my meaning.
Both words appear frequently in legal documents, often without definition. But just because they appear in a regulation or agreement, it doesn't mean you should [recommendation] use them in technical documents.
The main consideration for technical writers is to ensure that meaning is derived directly from the words you use rather than their context. If you leave it to contextual interpretation, you are just opening the door to misreading and ambiguity. The recommended usage is:
can = ability
could = potential
may = permission
might = possibility
must = necessity
should = recommendation
will = intention
would = conditionality
For example:
The applicant submits (no modal verb) [a statement of fact or an instruction].
The applicant can submit [the applicant is able to submit].
The applicant could submit [the applicant can submit but has choices].
The applicant may submit [the applicant is allowed (by an authority) to submit].
The applicant might submit [the applicant is able to submit but might not].
The applicant must submit [a statement of necessity or compliance].
The applicant should submit [the applicant is encouraged to submit].
The applicant will submit [the applicant will submit sometime in the future].
The applicant would submit if… [the applicant will submit if the condition is true].
If you are writing a compliance document, such as a code of practice or a procedure, define these words early in your Definitions section under the heading “Interpretation”. Your audience will thank you for making their task easier. Then search through your document and review how each word has been used. In particular, watch for when "may" is used as "ability" (change to "can") or "possibility" (change to "might") and whether "should" needs to be changed to "must". A typical interpretation section looks like this:
must – There is an absolute requirement to take the action
must not – There is an absolute prohibition on taking the action
should – The action is encouraged, but there might be valid circumstances for taking a different action
should not – The action is discouraged, but there might be valid circumstances for taking the action
might or may – The action is optional
Finally, you may [might] have noticed that "shall" is missing from the list. Except when used in the interrogative first person (Shall we go?), shall is considered old-fashioned and is disappearing from modern use. Either change to "must" or a more appropriate modal verb ("can" or "will", perhaps) or use a strong verb (the applicant is required to...).
For more information, see Grammar>Modal verbs in the Doxical Style Guide for Miners and Geoscientists.
