In one of my posts ("Farthest" vs. "The farthest") I said
Is there some difference in meaning? When would I use which?
I guess it is grammatical, though it does not seem to be idiomatic, according to Google Ngram
"Ngrams not found: When would I use which, When should I use each" indicate somehow people rarely say it that way.
However, if I had said this way
Is there some difference in meaning? When to use which?
The second sentence dose not seem grammatical.
What am I missing? Could someone please give a hint? Thanks in advance.