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I was taught that in English there is a rule that the subjunctive of verbs follows after verbs like suppose, assume, and so on. For examples,

  1. Suppose n be a number.
  2. Assume a fact be true.

But I have seen in (many) books and journals, this rule is (completely) ignored. People simply write as

  1. Suppose n is a number.
  2. Assume a fact is true.

I wonder if I am wrong and this rule does not exist or no longer exist.

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  • To me, it doesn't seem like the first two sentences use an infinitive: it looks to me like the subjunctive. Whether it's correct in those sentences, I can't say definitively. But to me, it sounds unnatural.
    – Kman3
    May 25, 2020 at 4:20
  • "Suppose n be a number" doesn't sound right to me. On the other hand, "Let n be a number..." is quite idiomatic. For 2, I would expect "Assume a fact to be true." The second versions of the sentences sound idiomatic in present-day English too. May 25, 2020 at 4:43

2 Answers 2

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The first two examples sound like they come from the eighteenth century. Where were you taught this, and with what materials?

Using the present subjunctive with such verbs is rather archaic these days, or at least affected. The usual marker for the subjunctive here is "that," although it may be omitted ("suppose that..."; "assume that...").

You do still hear the present subjunctive in phrases like "it's essential that the question be answered," though it seems to have persisted more strongly in the US than in England (I can't comment on other English-speaking places).

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In the second pair of sentences is omitted (is implicitly assumed) "Let's" (Let us):

  1. [Let's] suppose n is a number.
  2. [Let's] assume a fact is true.
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  • Not necessarily. Suppose and assume work just fine as imperatives by themselves. Even so, it's not clear how appending "let us" has any effect on the mood of the verb.
    – phoog
    May 25, 2020 at 4:36

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