“Skribi leteron” means “to write a letter”. In the interests of exploring Esperanto’s word building mechanisms, what happens if we do: “Leterskribi”? Is this allowed? What does this mean?
We sort of have something similar to this in English, “to letter-write”, meaning to perform a writing action where the writing is formatted like a letter. It is in fact allowed in Esperanto, and does have a slightly different meaning than keeping the words separated.
“Skribi leteron” implies the definite writing of a letter. Whereas “leterskribi” simply defines the type of writing one would perform if one were to perform that action. Whether that means you write one or several, or never finish the letter is not clear without an object.