Complex sentence: At first he maintained his refusal of Clairmont's companionship and allowed her to be in his presence only in the company of the Shelleys; later, they resumed their sexual relationship for a time in Switzerland.
Split into simpler sentences: At first he maintained his refusal of Clairmont's companionship and allowed her to be in his presence only in the company of the Shelleys. Later they resumed their sexual relationship for a time in Switzerland.