One is plural, referring to many different occasions. Examples: Sometimes I have meetings on Tuesdays. (This means that I have meetings on Tuesdays more than once) You should visit sometime! (This refers to one occasion --> You should visit at some day in the future) He rarely cries, but sometimes he cries when he is very sad. (Again, this is many future events. He will cry a few times in the future) Sometime in the future, there will be flying cars. (A single point. This means that flying cars will not happen at many different times in the future, but once. So, the speaker thinks humans will invent flying cars and keep using them forever).