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  1. I need to do this.
  2. I need do this.

My English grammar knowledge tells me that "need" doesn't have the same status as the modal verbs "may", "can", "should" and what not. Hence the second usage where two verbs appear consecutively is incorrect.

But yesterday, my native English speaking friends (Americans) told me that "I need compute this." is a perfectly grammatical sentence, and one is simply omitting the "to". How can this be? Is it a colloquial usage but grammatically incorrect, or is it grammatically correct? If it is grammatically correct, is it because "need" is a semi-modal verb?

EDIT: in particular, is it okay to use "need compute" in a scientific paper?

EDIT2: the exact phrase that raised the question was "The advantage of this representation is that we need only compute sums and products"

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Here's a web page with some links (I haven't gone there). We can say "All {I / you / they} need do is...", but not "I need do this". In specific: No, it's not okay to use "need compute" in a scientific or academic paper. Unless you have a specific sentence, however, the question is moot: two words in isolation don't allow a good question. –  user21497 Feb 6 '13 at 14:56
Please see this answer for more about the quasi-modals like need and dare. In contemporary English, modal need almost only ever occurs under inversion or negation. “One need not compute this” and “Need one compute this?” contrasting with “One needs to compute this.” –  tchrist Feb 6 '13 at 16:00
Related. –  RegDwigнt Feb 6 '13 at 16:53
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4 Answers

up vote 10 down vote accepted

Your friend is correct. "I need compute ..." is ungrammatical, but "I need only compute ..." is fine, if a little bit old-fashioned and formal.

Modal verbs do not use a "to". That is, you say

I can do this.

The verb "need" is a funny case; it is only modal in the negative. In the positive, we already have an equivalent modal verb; namely, "I must". However, there are two possible meanings for the opposite of "I must do this": "I am forbidden to do this" and "I am not required to do this". These two different meanings are conveyed by the modal verbs "I must not" and "I need not".

Searching with Google books, it appears to have been this way at least since 1600 (although back then, there was a positive construction "I must needs", which has since for the most part fallen out of use). Thus, you get various grammatical constructions.

In the negative, you have:

I need not do this.
I do not need to do this.

In the positive, you have:

I must do this.
I need to do this.
*I must needs do this. (obsolete)

"I need do this" is incorrect.

Over the last few centuries, "I don't need to" has slowly been replacing "I need not", but "I need not do this" is still used reasonably frequently, and is grammatical. However, if "I need do this" was ever grammatical, it was in the long distant past.

Finally, in the past you could say

it is not the case that I need compute this,

since that is a negative use (and this is why your friend might not be wrong). Today, I believe most people would use "need to" here. But if your friend was using "need compute" in the negative, there is a good case to make for it being grammatical.

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Very nice answer. Thank you Peter Shor. (I was not expecting to get an answer from one of the QC gods!) –  Memming Feb 6 '13 at 15:49
+1, though I would argue that even the last sentence does not sound grammatically correct, in modern English at least. –  ryan Feb 6 '13 at 19:48
@ryan: you're right, it sounds wrong to modern ears. But looking in Google books, you can easily find sentences such as "I do not think, however, that I need apologize for having done so" (1906), using the modal "need" in the same type of negative construction. So it was still in use not so long ago. –  Peter Shor Feb 6 '13 at 19:58
True, I've seen sentences structured that way, however, the construction of that sentence differs (slightly) from the construction of your last sentence. Also that is written English, which has remained much more formal then spoken English. Which of course is one of the reasons for confusion when it comes to English, there are differences between written and spoken English rules! dictionary.reference.com/help/faq/language/g20.html –  ryan Feb 6 '13 at 20:15
@jpillow: all these examples are either interrogative or (subtly) negative: e.g., need only ask means you need not do anything more than ask. Searching among the first 50 instances of "need take" in Google books before 1900, aside from the construction "had need take" (which isn't used nowadays), I only find one instance of need take that isn't negative in some way: "I should be obliged to you if you'll leave a Line or Two with Directions how to take the Things, and how long I need take them," from 1723. I'd say that need as a positive modal verb is currently ungrammatical. –  Peter Shor Feb 6 '13 at 23:20
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There's something missing from the description of the problem, which is the omission of the word only. I grant that it is an archaic construction, but I do not concede it is incorrect.

1.  In this setting, we need only consider X.

We could equivalently say

2.  In this setting, we only need to consider X.

I agree that the second is more common, but would submit that the first is not incorrect.

I'm adding a note forwarded by "friend #2". In fact, it appears that only does indeed affect grammaticality.

According to Swann (Practical English Usage, 3rd ed., § 366.2):

Need can also have the same present-tense forms as modal auxiliary verbs ... In this case, need is normally followed by an infinitive without to. She needn't reserve a seat - there'll be plenty of room. These forms are used mainly in negative sentences (needn't), but they are also possible in questions, after if and in other 'non-affirmative' structures. You needn't fill in a form.

Need I fill in a form?

I wonder if I need fill in a form.

This is the only form you need fill in. (BUT NOT You need fill in a form.)

So, it seems we can use need as a modal verb in an affirmative sentence when a 'non-affirmative' word (such as only, hardly, seldom etc.) gives the sentence a negative kind of meaning. Look at Swann's last example: the sentence becomes incorrect when only is removed.

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Sorry: substitue "compute" for "consider" to recover the original sentence. –  jpillow Feb 6 '13 at 16:19
But "only" wouldn't change the grammaticality of the problem. –  Memming Feb 6 '13 at 16:32
But why would the different position of only in the sentence alter the grammar of the sentence itself? Why would it be right to use need as a modal if you insert only between need and the following verb, whereas the same wouldn't be true when placing only before need? –  Paola Feb 6 '13 at 17:03
@Paola: adverbs like "only" go after modal verbs. So "you can only take one a day" but not *"you only can take one a day" (unless the meaning is "only you, and nobody else, can ..."). Thus, if "need" is a modal verb, and you do without the "to", you have to put the "only" after "need". –  Peter Shor Feb 6 '13 at 23:37
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"Need to" is one of the English modals. "I need to do this" is the more common usage and the one used most commonly in the States. "I need do this" is a bit affected use of the expression. Hope that helps.

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No, “need to” is not a modal, because modals do not take a to-infinitive complement. Rather, they take a bare infinitive complement. –  tchrist Feb 6 '13 at 15:58
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Simply put, your friends are wrong. Unfortunately, being a native speaker does not mean that one can speak a language correctly. Far from it. Indeed, it is often the case tat those who speak a language best are those who have learned it (very well) as a second language. They tend to be more aware of the grammatical rules. I remember a Scottish friend trying to convince me the following sentence is correct English:

Where has Dave went?

That said, your friends may be confused by an archaic idiom. For example, a character in Shakespeare's Hamlet says:

Such love must needs be treason in my breast.

This, however, is both archaic and different from needs compute. You could, if consciously trying to be pretentious say

[...] must needs be computed.

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