In fact, "S" means "se", wich we use for the the third person of the pronominal verbs, like "se lever" (to get up) for example. We use pronominal verbs in three cases : reflexivity, reciprocity or for idiomatic reason. This pronoun chenges depending on the person which is used with. It can be one of (me,te,"SE",nous,vous). Example : - Je me lève - Tu te lève - Il/Elle se lève - Nous nous levons - Vous vous levez -Ils/Elles se lèvent (the verb : se lever). Another example, when i say : "je me lave" (verb : to wash), it means like i am saying (i wash my-self) When we have a verb that starts with a voyel as those you have given in your question, we use only : m' ,t' , s' (when nous/ vous don't change). It's like "CI" in italian when we say (ci vediamo) for example.