I just edited this answer on unix.sx. The original sentence was
But it won't transform it to an other format.
I changed this to
But it won't transform it to another format.
The second form is standard, but is the first correct?
The string an other is vanishingly rare in English. In contrast another is positively pervasive. I think it would be fair to say that the second has eclipsed the first to the point of making the first unacceptable, even though it is a grammatical string.
Both an and another are members of the category of determiners, while other, on the other hand, is an adjective. There's no grammatical reason why DET + ADJ would unacceptable. So, it must simply be a matter of convention that makes an other unacceptable.
Historically, another and an other are simply two ways to write the same thing, and those dictionaries that are extensive in their example quotations may include an other as an example of another.
A similar word is cannot which is merely a way to write can not.
There is no difference in meaning between the two; all meanings of all sense of one are also a meaning of a sense of the other.
But history having moved on, we simply don't use an other much as we don't use mannes for man's, neyther for neither, and so on. As such your edit was correct.
About the one case I can think of where an other is to be favoured over another is if translating Rimbaud's "Je est un autre", I'd favour "I is an other" to "I is another", but bending the rules to put an emphasis on the word other in this way is acceptable precisely because one has already broken them in using the rule-breaking "I is…" to match the rule-breaking "Je est…".
In my opinion, just because "an other" is "vanishingly rare", that doesn't make its usage "unacceptable". In my situation, which is advising (via a letter) a candidate for an employment position who has not been chosen, it doesn't seem appropriate for me to tell him that "another" candidate has been selected, but it does seem appropriate for me to tell him that "an other" candidate has been selected. My aversion to "another" is because that seems to me to be saying that an additional candidate has been selected, rather than a different candidate being selected. Bottom line...that distinction is my "rule of thumb" too.
Here is a general rule of thumb: if you mean "a different [noun]", then it is more appropriate to use "an other"; if you mean "an additional [noun]", then it is more appropriate to use "another".
So in your example you should use "But it won't transform it to an other format."
Also take a look at Brett Reynolds' answer. It is good from a syntactical point of view.
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