He will return ______ a fortnight.

Concept

The preposition that goes with a span of time depends on what relationship to that span you mean. With a future action, “in + a period of time” means the action happens at the end of that whole period, i.e. once the period has passed. “within + a period” means before the period ends; “by” needs a fixed point in time, not a duration; “on” marks a specific day or date.

Application

A fortnight is a stretch of two weeks, i.e. a duration. The sentence describes a future return after that whole stretch of time has elapsed. The pattern “will + (verb) + in + (duration)” expresses exactly this: “He will return in a fortnight” = he will be back once the two weeks are over. “in” is therefore the natural fit.

Contrast

• “by” pairs with a fixed point (by Monday, by 5 o’clock), not with a duration such as a two-week span, so it does not fit here.

• “on” marks a specific day or date (on Tuesday, on the 5th); a fortnight is a length of time, not a single day, so it clashes.

• “within” is grammatical too, but it expresses a different relationship: it gives only an upper limit (no later than the two weeks), leaving the action free to fall anywhere earlier inside the span rather than measuring the action against the full stretch. That ‘no later than’ reading is a distinct meaning from marking the action by the two-week duration itself.