can / may / might / must / should

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Always be as precise as possible. The incorrect or vague use of the terms can, may, might, must, should, could, and so on is one of the most frequent causes for misinterpretation.

 1  Use can when you mean the ability or power to do something.

 2  May and might both indicate possibility or probability. Might suggests a somewhat lower probability than may.

Don’t use may to imply the ability to do something. In this case, use can.

Don’t use may to imply the permission to do something. In this case, use allowed to.

Tip:
Phrases with you include can more often than may (“You can ….”).

 3  Use must to describe a user action that’s required. If you feel that the word must is too strong or impolite because it implies an obligation, rephrase your sentence as an instruction or use need to or have to.

Note:
In American English, don’t use shouldn’t instead of mustn’t because you think that mustn’t sounds too British. Use must not instead, which is unambiguous for an international audience.

 4  Use should only to describe a user action that’s recommended but optional. However, try to avoid the word should altogether because it always conveys an element of doubt. Instead, clearly tell the reader what to do, or clearly mark your sentence as a recommendation.

Never use the word shall in technical documentation.

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Examples for the use of “can”    1 

No:

You may use a spoon or a fork.

Yes:

You can use a spoon or a fork.

No:

You may use the program to write a manual.

Yes:

You can use the program to write a manual.

Examples for the use of “may” and “might”    2 

Yes:

You may be right.

Yes:

Print quality may be poor if you use cheap paper.

Yes:

The camera is shock proof, but it might break if you drop it from a height of more than 1.5 meters.

No:

The remote control may also use a frequency of 140 MHz.

(This is Ambiguous: It could mean that (a) you’re allowed to use a frequency of 140 MHz; (b) the device is able to send at 140 MHz if you set it up to do so; (c) it might happen that the device sends at 140 MHz at its own discretion.)

Yes:

It’s also allowed to use a frequency of 140 MHz for the remote control.
(If you mean that law permits you to use this frequency.)

The remote control can also use a frequency of 140 MHz.
(If you mean that the device is able to send at this frequency.)

The remote control might also use a frequency of 140 MHz.
(If you mean that the device chooses the frequency automatically.)

No:

You may print the report, or you may save it to a file.

Yes:

You can print the report, or you can save it to a file.

Examples for the use of “must”    3 

Yes:

The fluid must pass the valve.

Yes:

If the function doesn’t work, you must contact support.

Top:

If the function doesn’t work, you need to contact support.

Top:

If the function doesn’t work, contact support.

Examples for the use of “should”    4 

No:

You should make a backup copy of the file.

Yes:

We recommend that you make a backup copy of the file.

Top:

Make a backup copy of the file.

No:

You should get a reply within 24 hours.

Yes:

You will usually get a reply within 24 hours.

Top:

We usually reply within 24 hours.

must / must not

For many writers who speak English as a second language, the use of must and must not is a dangerous pitfall. In some languages, the literal translation of must not means need not. In English, must not and its contraction mustn’t mean that there’s an obligation not to do something.

Yes:

You must not smoke while filling up the tank.

Yes:

You must not use a 110-volt device on a 220-volt outlet.

cannot / can not

Use cannot or its contraction can’t if you mean is not able to. Here, the word not negates the word can.

Use can not if the word not relates to the action following the word can, not to the word can.

 

Yes:

You cannot / can’t start the compressor while the engine is running.

(means: It isn’t possible to start the compressor while the engine is running. It won’t work.)

Yes:

You can not start the compressor while the engine is running.

(means: You don’t have to start the compressor while the engine is running. It’s not required, although it may be more common or recommended.)

can / could

The word could conveys an element of doubt, which is something that you must avoid in user assistance (see Be specific). For this reason, don’t use could when you mean can, does, or will.

Only use could as the past tense of can. In most cases, however, it’s better to use the present tense (see Use the present tense).

 

No:

If you follow this rule, this could improve your documents.

Yes:

If you follow this rule, this improves your documents.

No:

Instead of using the menu, you could also use a keyboard shortcut to format the text.

Yes:

Instead of using the menu, you can also use a keyboard shortcut to format the text.

Yes:

The program couldn’t find your name in its database.

Top:

The program can’t find your name in its database.


Be specific