Last spring, Mario spent it sending letters, pretending to be someone he wasn't.
Does spent it imply that Mario spent that whole spring sending letters? Or not necessarily?
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Sign up to join this communityLast spring, Mario spent it sending letters, pretending to be someone he wasn't.
Does spent it imply that Mario spent that whole spring sending letters? Or not necessarily?
Your sentence--Last spring, Mario spent it sending letters, pretending to be someone he wasn't--doesn't work, because "last spring" is an adverb; "it" doesn't refer to "last spring." "Spend" is a verb that takes an object.
You need something like:
"It was spring. Mario spent it sending letters . . ."
Or you can say:
Mario spent last spring (or: an entire week last spring) sending letters, pretending . . .
In these later two rewrites, spend has it object "it" (which refers back to spring) or "last spring."
As another answer suggests, "sending letters" is an important activity but it doesn't need to be (can't be) the only thing Mario does during the spring or during a week in the spring.
The implication with "spent the spring doing something" is that the thing you were described as doing was one of the more important/time-consuming things you did during that spring. It also implies that it was done relatively frequently throughout the spring. I wouldn't say that the whole of Mario's time during that spring was spent sending letters, though.