Krashen says that the two important systems we use to learn a language are acquisition and learning, and that these two systems are different but related. The acquisition system is a largely unconscious activity and is the one we use when learning our first language as children: we listen and speak, but we are more concerned with communicating meaning than with our sentence structure. As a result, those students who speak more in the second language without worrying about a perfect sentence structure are likely to learn faster. Krashen believes this acquisition system is more important than the learning system, which refers to formal study of another language using grammar books.
Krashen believes that monitoring is a process we use during language study that links the acquisition and learning systems. Students "monitor" their speech by applying grammar and syntax rules learned in class. Initially it takes the student more time to monitor what they are saying and they need time to construct sentences; as they progress, less time is needed. Students who monitor their sentence construction too much have trouble progressing because they inhibit the acquisition system of learning. Students who find a balance between speaking and monitoring progress faster because they are more likely to engage in conversation and thus use the acquisition system more effectively.
Another element of Krashen's theory is that successful language learners follow an instinctive, or natural, process that transcends factors thought to help language acquisition, such as age and learning conditions. He believes that they acquire single words and string them together without regard to syntax; in other words, they focus more on meaning. They also identify the key ways of beginning and ending sentences in the second language, and also look for ways of rearranging sentences to form questions. Krashen also states that natural order learners benefit from frequent communication with speakers who are either native speakers of the second language, or others more fluent in it as a second language.
The final component of Krashen's theory relates to factors that are not directly connected to language learning but directly affect success. These are primarily motivation, self-confidence and anxiety. Lack of confidence and nerves inhibit students and it is noticeable that the most successful language students are relaxed and self-confident. Motivation is also a factor, but even students that are motivated will not learn well if they are not relaxed about it.