【正文】 ion and learning are used in second language performance .Krashen holds that our acquired petence guarantees us to have the ability to produce utterances in another language .The learned system has a special function—serve as a Monitor. It can help us correct or change the output of the acquired system before we speak or write. Learners who use their acquired knowledge to say “He is going” can check against their learnt knowledge whether “he” is the appropriate pronoun whether the present continuous is the appropriate tense ,whether “is” should agree with “he” ,and so on .However ,the Monitor cannot be used at all the times. The real application of Monitor needs three conditions: 1) Time: Learners need sufficient time to think about and use the rules available to them effectively in their learned system. For most second language learners, normal conversation does not allow enough time to think about and use rules. 2) Focus on form: Although time is not enough, one must also focus on form or think about correctness when he uses the learned knowledge Even when we have time, we may be so involved in what we are saying that we do not attend to how we are saying it. 3) Rule knowledge: In order to use the rules, the learner has to know them .That is to say, one has to have an appropriate learned system in order to apply it. Input HypothesisThe Input Hypothesis is central to Krashen’s SLA theories. First, attempts to explain how we do acquire language. Krashen gives us “Humans acquire language in only one way—by understanding messages or by receiving “prehensible input”.That is to say, language acquisition depends on trying to prehend what other people are saying .The learner should focus on meaning ,but not on form .If a learner hears a meaningful speech and makes efforts to understand it ,acquisition will occur . If a learner concentrates on the forms of language ,then acquisition will fail .Second, it also attempts to explain how a learner progresses in acquisition Krashen provides us with a model like this: “We progress along the natural order by understanding input that contains structures at our next stage—structures that are a bit beyond our current level of petence.” Suppose the current level is “i” ,the next stage should be“i+1”(stage able beyond the current level).Learners can understand language containing un acquired grammar with the help of context, including extralinguistic information, knowledge of the world ,and previously acquired linguistic petence . Hypothesis“Affective filter” is a mental block that prevents acquirers from fully utilizing the prehensible input they receive for language acquisition. That is to say, in order to acplish the task of acquisition ,prehensible input is only a necessary condition ,but it alone is not sufficient ,the acquirer needs to be “open” to the input .People should not put the acquirer “on the defensive” ,because when the affective filter is up, the acquirer may understand what he hears or reads, but the input will not reach the language acquisition device. This occurs when the acquirer is unmotivated, lacks selfconfidence or feels anxious .The effective way to lower the affective filter is either to make him care nothing about the failure in language acquisition or to consider himself to be a potential member of the group using the target language. The above are the five famous hypotheses in the field of SLA. Krashen stated: We can summarize the five hypotheses with a single claim :people acquire a second language only if they obtain prehensible input and if their affective filters are low enough to allow the input ‘in’ .When the filter is “down” and appropriate prehensible input is presented ,acquisition is inevitable.2. Current Situation and Main Problems of English Reading Teaching in Senior High School Students’ Current Situation and Main Problems Unfamiliar to Vocabulary and GrammarVocabulary is the key to the reading. The main problem of reading is the vocabulary and its significance. The small vocabulary cannot expand the reading quantity, narrow to lexical meaning39。s understanding, so it will have difficulty to prehend author39。s true meaning. Therefore, vocabulary has the important influence to raise the reading speed and the understanding degree. Vocabulary is the basic knowledge of English language, even the basic knowledge and premise of English reading. Students’ biggest obstacle is so many new words that make reading difficult. When students do reading exercises, if they met too many new words will lose their reading interest and confidence. Visible vocabulary shortage in reading is the main problem. Many students will anxiously when they meet the new word, even worry about that they could not understand the entire chapter article. In daily reading process, the student will always meet some special sentence type and the quite plex sentences .This kind of sentences show that the sentence is long, the subordinate clauses are many and the sentence type is special .When students meet long sentences, they do not clear sentence structure, and cannot understand the sentence means. Encounter pound sentence, it is hard to understand the meaning and it is easy to cause error. Each kind of syntax phenomena and abbreviation sentence has the, tremendous influence for understanding the sentences. This indicated that the plex sentence type has the varying degree influence to student39。s reading .For this, the true reason is the student cannot grasp the grammar knowledge, cannot make the correct analysis to the whole sentence firm, thus it affects students to hold the whole sentence meaning. Lack of Background KnowledgeInvestigation, the author known that many students mastered a large vocabulary and a plete set of grammar knowledge, but there also will be many difficulties in reading prehension and linguistic expressions .The immediate reason cause these questions is the students lack of the west cultural backgro