I was at one of Stack Exchange's many forums, and I came across a question that made wonder on whether I was building a sentence correctly when using the word "Why", and if that new way was right.
How I would write it:
"Why not to use a cotton base layer?"
How I saw it written:
"Why not use a cotton base layer?"
My question is, which sentence is correct, why, and if both, are they interchangeable?
Both sentences are correct but convey opposite messages.
"Why not to use a cotton base layer?" is a question followed by reasons why you should not use a cotton base layer (e.g. "Because a cotton base layer is no adequate.")
"Why not use a cotton base layer?" however, is a rhetorical question usually followed by reasons why you should use a cotton base layer (e.g. "Because cotton is the perfect material.")
In your original post, the second sentence is usually the one that is used, and probably the one you are looking for.
Alternatively, you could rephrase the first one : "Why should you not use a cotton base layer?", and the second one : "Why don't you use a cotton base layer?" Of course both of those can use other subjects ("Why shouldn't he", "Why don't we" etc...)
The first construction does not read like a real question. As mentioned in the comments, it would usually be followed by reasons why one should NOT [verb]. (Interestingly, if spoken, the word not in such a case would be emphasized, as if to cue that it's not a real question.) If the first meaning is intended, it would seem less awkward to headline with "Reasons to not use cotton base layers" or "Why you should not use cotton base layers".
