Can I just omit "to" at the end of this sentence?
Native speaker’s answer
Rebecca
No, it would be weird if you were to omit "to" here and just say "when you need them", because "when you need them" indicates a different meaning. The preposition "to" here is a shortened version of "when you need them to appear and disappear", however "appear and disappear" are already implied at the beginning of the sentence and so it is not necessary to repeat again.