In other words: Voiceless stops ([p], [t], and [k]) are aspirated ([ph], [th], and [kh]) when they occur immediately before (no sound in between) a stressed vowel, and there is no [s] in front of the voiceless stop. So, they are not aspirated after [s], if they occur before an unstressed vowel, or if there is a liquid or glide between the stop and the vowel (and then the liquid/glide is considered voiceless = [r ])