>I was taught that the phrase "I cannot help but think" is grammatically
>incorrect, and that "I cannot but think" or "I cannot help thinking" were
>correct.

>Can somebody explain to me exactly WHY this is so?
Well, I can't speak for your teachers, so I can't say why you were taught that way.
Sorry, you'll have to take it up with Miss Fidditch.

The constructions you mention are weird enough, though, to authorize plenty of righteous grammatical indignation, if there's any to spare. That is, they're all idiomatic, and we all know that idioms are evil, right? Unless they happen to be common, of course, and then they're, well, obvious.

The constructions at issue are:

  1. I cannot but think (that S)-- where S is some sentence
  2. I cannot help but think (that S)
  3. I cannot help thinking (that S)
Let's turn cannot into can't so it sounds like English -- nobody ever says cannot, they write it. With this proviso, I'd opine that No. (1), I can't but think stinks; and the same is true of the cannot form, so that's one bum steer Miss F. left you with. Do not say or write I cannot but think ..., then. One down.

That leaves Nos. (2) and (3), and (3) is very straightforwardly OK (with can't, that is).
So Miss F. is batting .500 so far.

(2) is maybe a little precious -- I mean, what's the but doing for you with an infinitive in (2) that a simple conjunctionless gerund doesn't do in (3), anyway? I'd rank it as a parvenu construction, and I'd expect to find it in a paper submitted by one of the legion of undergraduates who have figured out that the more words you use, the heavier the so-called thought you're expressing. But I'd red-pencil it for prolixity.

What you have here falls into the realm known technically as Negative Polarity; the metaphor is based on the phenomenon of electric and magnetic fields and their poles, though there is as yet no analog of Maxwell's Equations for negation. Basically, there are a whole lot of idioms, constructions, lexical items, and even funny phenomena that seem to be possible in English only within a "negative field".

For instance, the word budge is seldom used in the affirmative; and neither is fathom [as a verb meaning 'comprehend'], nor the phrases a red cent, can seem to, or any more, nor the verbs last or take with long, or, specifically, neither can the construction can help with a gerund. Exx:

The silly sentences above are what happens when one follows syntactic rules blindly; of course, nobody (*somebody) ever does that, except for us linguists, who can be found testing syntactic rules to destruction in our linguistic labs, in Chinese restaurants, and everywhere else. That's how we find phenomena like Negative Polarity. Individual members of the congeries of words, phrases, constructions, exceptions, contractions, and generally weird phenomena are known as "Negative Polarity Items", or NPI's for short.

You can think of NPI's as idioms, but that's just a confession of incomprehensibility; they're all strange. The sense of

I can't help (but) think(ing) (that S)
is
I am not able to abstain from thinking/the thought (that S)
which might be construed either as an generic event:
I'm always thinking about (the fact that) S, though I'd rather not do that
or as a stative result of some mental sort:
I am forced to the conclusion that S, though I'd rather not believe it
I am forced to the experience of S, though I'd rather not have it
because think is ambiguous between a (mental) action (conceptually a verb, whence do) and its resultant mental state (conceptually an abstract noun, whence it).

The I can't help thinking construction seems to be ambiguous between these two senses:

I can't help thinking he was hiding something.
I can't help thinking about Bosnia.

To me, the I can't help but think construction seems more oriented toward the first, rather than the second, sense:

I can't help but think he was hiding something.
?I can't help but think about Bosnia. [...think what about Bosnia?]

Which makes sense, since the first one has an actual clause (an S, a proposition) involved, whereas Bosnia is a label, not a proposition.

In any event, Official Grammar of the sort that the Miss Fidditches of this country invoke has nothing useful to say about the Correctness of any of these constructions, since they're idiomatic and therefore have Grammatical License.

So let your conscience guide you, and don't budge.

  - John Lawler       Linguistics Department and Residential College     University of Michigan
    "Language is the most  massive  and  inclusive  art  we know,  a           - Edward Sapir
      mountainous and anonymous work of unconscious generations."       Language (1921)
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