Translation:The man often wears short pants in the summer.
Hmmm... good question, never really thought about it... Seems like they do, but shouldn't: http://ca.complex.com/style/2013/05/reasons-why-men-should-never-wear-capri-pants/
As I understand it, dum might imply "for the duration of the summer", like he put the shorts on in June and didn't take them off until September, which would be a couple of levels of funky.
That or it's just that en somero is one of those idiomatically fixed expressions that just has to be remembered.
The Esperanto sentence says "mallongan panatalonon", which is in the singular form. From what I know so far, if there's no article in front of a noun phrase ("la" is the only one I know) then it can either be translated as having no article ("[one pair of] short pants") or having "a" in front of it (a [pair of] pants). So because the phrase is singular and not plural, must the Esperanto sentence given be taken to mean "The man often wears one particular pair of pants in the summer"? If it were plural ("mallongajn pantalonojn") than of course it could mean he wears multiple pairs.
I don't believe we've run into this issue elsewhere so far, because usually we were definitely talking about noncounted verbs, like "Li trinkas kafo.", or a definite amount of a counted verb: "Li portas ĉemizon." or "Ŝi trinkas tazo da suko.".
One just doesn't wear "a pair of" pants in Esperanto: "unu pantalono" = one pair of trousers. Just as in Dutch a "a pair of scissors" = "een schaar" and "a pair of glasses" = "een bril". The English language defines them as a pair of two objects, Esperanto and Dutch define them as a single object (except "okulvitroj" in Esperanto for a pair of glasses).
I carefully read the post and found one question:
So because the phrase is singular and not plural, must the Esperanto sentence given be taken to mean "The man often wears one particular pair of pants in the summer"?
Answer: no - no more than "I wear a coat in the winter" means "I wear the same coat all winter long."
As a general principle, just about anything you can say about place can be said about time. It's a metaphor - and a very basic metaphor to how we think. There are some subtle differences at time, but to have something happen "in" a certain time period is perfectly natural - and Esperanto routinely uses en in this way.
The difference between en somero and dum somero is that en implies that something happens within that time period, while dum suggests an event or condition which lasted the entire time period.
- Mi portis ŝorton en la somero. I wore shorts in the summer.
- Mi portis ŝorton dum la somero. I wore a pair of shorts for the summer.