>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.
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:
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:
You can think of NPI's as idioms, but that's just a confession of incomprehensibility; they're all strange. The sense of
The I can't help thinking construction seems to be ambiguous between these two senses:
To me, the I can't help but think construction seems more oriented toward the first, rather than the second, sense:
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.