The Monitor hypothesis explains the relationship between acquisition and learning defining the influence of the latter on the former. According to Krashen, the acquisition system is the initiator of the utterance, whereas the learning system performs the role of the “monitor” and assists in editing and correcting of the utterance. The linguist further distinguishes between three types of monitoring behaviour in learners: over-users (those who use the “monitor” all the time); under-users (those who haven’t learned or who prefer not to use their learned knowledge); and optimal users (learners who use the “monitor” appropriately).