Within vs in. I’ve noticed many writers using within when the simpler in would be preferable. Why do so many writers confuse within and in? I suspect that some people like to use a more fancy word because they believe it makes the writing sound more professional or academic. However, the point of good writing is clarity of ideas, so using a fancy word when a plain one will do is not good writing.
The reason we have similar prepositions, like in/inside in/into, on/onto and in/within is for the purpose of speaking with precision. The simpler word has the the simpler meaning; the more complex word has a more specific meaning. So the difference isn’t a matter of right or wrong; it’s about the necessity for precision.
We use within when we need to be clear about boundaries. If I say “I have to complete this editing in two days” the implication is that I might be editing continuously for two days. However, “I have to complete this editing within two days” explicitly places the completion time before the deadline. When we are simply writing about location, then in is preferable: “I have to complete this editing in my office because my home computer is down.” Here’s a discussion of the difference on the English Language Blog.
On vs onto; into vs in
Other prepositions of place follow similar usage patterns. The compound preposition (into, ontop) puts more emphasis on boundaries or transition. For example, “Move that rock onto the pile.” The focus of the sentence is on the movement. Conversely, “Put that rock on the pile.” The focus of the sentence is on the end result. Similarly, “I saw him put it in his pocket” is acceptable, but “I saw him put it into his pocket as he distracted you with his hat” clearly focuses on the action within a constrained time.
The bottom line: in and within
Use in in all cases unless you are emphasizing the boundary. For physical locations, that’s rare. For time, it’s more common.