first stage, preproduction, is also known as the silent period. Learners at this stage have a receptive vocabulary of up to 500 words, but they do not yet speak their second language. Not all learners go through a silent period. Some learners start speaking straight away, although their output may consist of imitation rather than creative language use. Others may be required to speak from the start as part of a language course. For learners that do go through a silent period, it may last around three to six months. The second of Krashen's stages of acquisition is early production, during which learners can speak in short phrases of one or two words. They can also memorize chunks of language, although they may make mistakes when using them. Learners typically have both an active and receptive vocabulary of around 1000 words. This stage normally lasts for around six months. The third stage is speech emergence. Learners' vocabularies increase to around 3000 words during this stage, and they can communicate using simple questions and phrases. They may often make grammatical errors. The fourth stage is intermediate fluency. At this stage, learners have a vocabulary of around 6000 words and can use more complicated sentence structures. They are also able to share their thoughts and opinions. Learners may make frequent errors with more complicated sentence structures. The final stage is advanced fluency, which is typically reached somewhere between five and ten years of learning the language. Learners at this stage can function at a level close to native speakers. Krashen has also developed several hypotheses discussing the nature of second language learners' thought processes and the development of self-awareness during second language acquisition. The most prominent of these hypotheses are Monitor Theory and the Affective Filter hypothesis. Kleine and Perdue stages From the early 1980s, a large research project into SLA was carried over at the Max Planck Institute for Psycholinguistics, headed by Wolfgang Klein and coordinated by Clive Perdue, which studied Second Language Acquisition by Adult Immigrants coming into Europe. The results, published in the early 1990s, proposed that second language acquisition proceeds along three stages: pre-basic variety (or nominal utterance organisation), basic variety (or infinite utterance organisation), and post-basic variety (or finite utterance organisation). The pre-basic stage is usually very short, with a small lexicon of nouns and no verbs. The basic stage sees the introduction of verbs in their