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I have a sentence which is, admittedly, a bit old-fashioned... however, I am wondering if it's a reasonable use of "need" as a modal auxiliary verb

Note that, because H is real and symmetric, we need store only half of the matrix.

I could add "to" between "need" and "store", but to me that feels like a ever-so-slightly different meaning. Adding "to" implies (to me) a kind of absolute necessity, that "we need to do something", while my sentence structure has a bit more of the flavor of "if we store H, then we need to store only half..."

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  • This may be a duplicate of english.stackexchange.com/questions/297233/…
    – alphabet
    Mar 14, 2023 at 4:24
  • It's related, for sure. Intuitively, "need store only" and "need only store" don't sound equivalent to me -- "need only to store" vs. "need to store only"?
    – kc9jud
    Mar 14, 2023 at 4:29
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    You mean "we only need to store half the matrix", right? If so, then you are correct in saying that adding "to" implies necessity. The reason is that "need" without "to" is a negative polarity item (i.e. it works only in negative contexts) and "only" makes a negative context. So "We need only store half the matrix" is correct. However, in "We need to store only half the matrix", the scope of the negative context starts with "only"; "need" is outside that context and so implies, as you said, necessity. This sentence implies that you cannot store the full matrix.
    – Rosie F
    Mar 14, 2023 at 6:42
  • Whether you accept it depends on whether your grammar accepts only half of the matrix as a negative context. If so, fine; if not, ungrammatical. Try it without only to see. Mar 14, 2023 at 14:12
  • It seems to me that you “must” store a matrix or some portion thereof for every letter, so it doesn't matter if need to sounds bossier than need only (which I'm not convinced that it does). We need store half of the matrix only sounds about the same as We need to store half of the matrix only — to me. Mar 14, 2023 at 17:17

2 Answers 2

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Yes, it's perfectly acceptable. This use of "need" (followed by a bare infinitive) is often called a "pseudo-modal". Other pseudo-modals include "dare", "ought", and "used". They have some, but not all, properties of the typical modal auxiliaries.

Others have suggested that this "need" needs negative polarity, but that isn't always required. For example:

Do we need to remind you that cheating is forbidden?

can become

Need we remind you that cheating is forbidden?

You are right that this can seem somewhat old-fashioned, but in some contexts it sounds entirely natural. I find your sentence to be fine.

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Yes, "we need only store half" is a reasonable use of modal "need", licensed by the negativity of "only".

You are right that adding "to" changes the meaning. In fact, for example, "You don't need to go", "You need to not go", and "You need not go" have 3 different meanings.

(I think you probably mean "we need store only half", but what you really mean is up to you.)

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