Complex sentence: The monitoring hypothesis posits that social facilitation effects do not occur when the performer is familiar with the observers or the situation because the performer will know how the observer will act or how the situation will take place.
Split into simpler sentences: The monitoring hypothesis posits that social facilitation effects do not occur when the performer is familiar with the observers or the situation. This is because, in those cases, the performer knows how the observer will respond or how the situation will take place.