In technical writing, you often have to choose whether to begin a sentence with "if" or "when".

If / When you need to ... , do ...

If / When you need to specify a year, use YYYY-MM-DD format.

However, although in most cases both options are fine, there are also cases when I have a feeling that only one of these words sounds good.

When they are used as objects and not subjects, omit relative pronouns which, who, whom, whose, and that.

Is it correct to say that to make things more clear to myself, I should read "If ..." as "In case ...", while "when" as "In cases when ...", and use this method to choose between them? Notice the case vs. cases case.

1 Answer 1


Your way of thinking about them seems correct.

"If" would be used for cases where it is unclear whether or not something will happen.

"When" would be used for cases where something is going to happen but won't happen in every case.

Compare for example:

"If you need to specify a year use this format"

It is unknown whether you will need to specify a year but if you do have to then use this format.

"When you need to specify a year, use this format"

You will at some point, or multiple times, need to specify a year and when you do you, use this format

In practice the difference can be quite small and you will likely hear people use them somewhat interchangeably

  • Thanks. I still cannot understand why the word "If" doesn't play good in the third example; probably I need to ask another question about it ...
    – john c. j.
    Jun 11, 2021 at 23:23
  • 2
    For the third example, it is known by the author that whatever they are talking about will sometimes be used as objects and is instructing you what to do when this happens so it follows from the logic in my answer that "when" is the appropriate option. Let me know if that makes sense Jun 12, 2021 at 6:02

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