I'm having a slight dispute with a friend who proofreads my paper on the correct tense in this sentence:

One of the key requirements for the web app was that it must automatically work with any new lexicons that might be added later.

As the main clause is in past tense, do I have to change the present tense of "must" to something else? And how?

To clarify, the requirements were written down before the app development begun (which is in the past). And the lexicons might be added after development is finished (which is in the future).

  • The verb of "must automatically work" is "work," not "must." If any word would change tense, it would be "work." "must" here is a so-called 'helper' verb augmenting the modality or mood of "work." You could use the verb phrase "must have automatically worked" if you want it to be past tense. Commented Jul 30, 2017 at 13:35
  • @BillJ Eh? Must is the 'past tense' of must, not of might, which is the 'past tense' of may. Commented Jul 30, 2017 at 13:56

3 Answers 3


The past tense of must is had to. That is, must is an auxiliary verb, which can't be used in every grammatical situation. When you put it into the past or the future, or combine it with another auxiliary verb, you replace must with the periphrastic equivalent have to.


One of the key requirements for the web app was that it had to automatically work with any new lexicons that might be added later.

See Wikipedia. Most modals have several different meanings, so these specific replacements don't always work. But for the relevant meanings, Wikipedia says that:

  • must gets replaced by has to,
  • may and might get replaced by is allowed to,
  • can and could get replaced by is able to,
  • will gets replaced by is going to,
  • should gets replaced by is supposed to.

EDIT: On the other hand, now that you've clarified that the must applies to the future, and not the past, it is clear that you don't need to use a past tense verb. It's fine as is. (On the other hand, past tense had to wouldn't be wrong, either.)

  • Peter, I trust your rep, but you really should include a reference that backs this up.
    – vpn
    Commented Jul 30, 2017 at 15:16
  • I was the downvoter and left a comment ;) Will retract now that you've added references.
    – vpn
    Commented Jul 30, 2017 at 17:16

There is absolutely nothing wrong with the stated sentence. It is possible that this requirement existed before, but was later amended or removed, and that this sentence explains the presence of certain functionality in the finished product.

In describing what that requirement was, there is no need to change the tense of the requirement, since the requirement most likely did not specify the feature to stop working at some point in the past.

  • Why is that button green? I thought they should all be yellow.
  • Ah, we missed that one button! One of the original requirement was that all buttons must be green, but the customer changed their mind.

There's nothing wrong with the phrase from a grammatical stance, but from a stylistic stance I'd prefer to use the present tense. Isn't this still a requirement of the app, even if it used to be one too? Current phrasing seems to imply that this is no longer a requirement but it used to be one. Not ungrammatical but less precise.

As far as changing "must" the problem is that "should" and "ought" imply some wiggle room whereby the requirement is that the app should strive to have the feature, but it's not critical that it does have the feature. "Must" implies that the requirement is that the app has the feature. You lose semantic content by using a modal other than "must."

  • If you're talking about the process of designing and developing the app, you want to use was. If you're talking about the app in general, you want to use is. The clarification from the OP seems to indicate that was is the right tense here. Commented Jul 30, 2017 at 16:40

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