I don't understand why "gli" is translated as masculine singular "he". Wouldn't it be "they". The literal translation would be something like "Swimming is pleasing to them in the summer"?
Gli piace nuotare d'estate = A lui piace nuotare d'estate that in english become He likes swimming/to swim in the summer.
In the spoken language, Gli can subsitute A loro. It is widely accepted.
In the written language, I don't really know to which extent Gli can substitute A loro.
Read Flavio's answer: "gli" == "a lui" -> "to him". To him the swimming is pleasing.
Thanks for the response. I'm not sure if "a lui"-> "gli" then "a lei" -> "le"(plural form)?
see http://tutorino.ca/grammatica/2006/12/26/italian-indirect-object-pronouns.html You can say a lei piace, piace .... a lei, or le piace. So, yes, a lei -> le but that is singular, not plural.
Now i see i have to learn the differences between direct and indirect objects. - I'm poor at grammar even in English. Again, thanks for the kind response!