In this article I will try to discuss second language acquisition theories: compare and contrast L1 and L2 acquisition, introduce some key points, such as internal factors which include innatism/universal grammar, internal theories, to be specific I will discuss them in Krashen's acquisition, monitor, input, affective filter ways. Additional, as a language learner I will describe my own beliefs about language leaning.

First of all, research on language acquisition can be divided into first and second language learning settings. Language is not a mental phenomenon, it is behavior. Like other forms of human behavior, it is learnt by a process of habit-formation, in which the main components are. Activities require mature thought processes which stimulate and motivate adult minds Adults generally need patience and repetition to solidify new language concepts or skills. For myself learning, I enjoy materials that relate to my personal experiences and interests, and also tend to have a lot on my minds and limited time to practice English outside the classroom. Second language acquisition is a theoretical and experimental field of study which, like first language acquisition studies, seeks to understand the phenomenon of language development, in this case the acquisition of second languages. More recently, studies have shown that there are similarities and differences between first and second language learning. Often, our increased knowledge of first language acquisition has served as a backcloth for perceiving and understanding new facts about second language learning. In addition, first language learning is most relevant to child development while second language learning pertains primarily to adult learning, although most general theories of language learning apply to both. While it is not clear whether different psychological processes are involved in first and second language learning, there are differences in the way children and adults learn and this has important implications. Theories of adult learning and literacy are more likely to provide an appropriate framework for second language learning compared to those concerned with child development. It seems obvious that the advantage for adults is that having developed all the skills necessary to learn a language, such as understanding semantic relations and grammatical sensitivity develop with age. Especially in the areas of vocabulary and language structure, adults are actually better language learners than children. Older learners have more highly developed cognitive systems, are able to make higher order associations and generalizations, and can integrate new language input with their already substantial learning experience.

Secondly, this is a brief description of Krashen's widely known and well accepted theory of second language acquisition. Krashen's theory of second language acquisition consists of five main hypotheses: Acquisition-Learning, Monitor, Natural Order, Input, and Affective Filter.

The Acquisition-Learning distinction is the most basic of all the hypotheses in Krashen's theory. According to Krashen there are two ways to develop knowledge of second language: "Acquisition" and "Learning". Acquisition is the product of a subconscious process very similar to the process children undergo when they acquire their first language. It requires meaningful interaction in natural communication where the focus is on meaning. "Learning" is the product of formal instruction and it comprises a conscious process which results in conscious knowledge about the language, for example knowledge of grammar rules.

The Monitor hypothesis is focus on the relationship between acquisition and learning and defines the role of grammar. According to Krashen, the acquisition system is the utterance initiator, while the learning system performs the role of the "monitor" or the "editor". The "monitor" acts in a planning, editing and correcting function when three specific conditions are met which means the second language learner has sufficient time at learners focuses on form or thinks about correctness, and they knows the rule.

The Input hypothesis is Krashen's attempt to explain how the learner acquires a second language. So, the Input hypothesis is only concerned with "acquisition", not "learning". According to this hypothesis, the learner improves and progresses along the "natural order" when learners receive second language "input" that is one step beyond learners' current stage of linguistic competence.

Finally, the Affective Filter hypothesis, that a number of 'affective variables' play a facilitative, but non-causal, role in second language acquisition. These factors include: motivation, self-confidence and anxiety. Krashen claims that learners with high motivation, self-confidence, a good self-image, and a low level of anxiety are better in second language acquisition. Low motivation, low self-esteem, and debilitating anxiety can combine to 'raise' the affective filter and form a 'mental block' that prevents comprehensible input from being used for acquisition. In other words, when the filter is 'up' it impedes language acquisition. On the other hand, positive affect is necessary, but not sufficient on its own, for acquisition to take place.

Moreover, Universal grammar is a theory of linguistics postulating that all languages have underlying principles of grammar. These underlying principles are said to be innate to all human beings. Studies show that the order in which a language is learned by children in terms of syntax and morphemes, for example is highly similar in many cases between L1 and L2, that in fact the so called errors that a child makes in learning English as a second language are similar to those of a child learning English as L1. In fact, even if the UG hypothesis is correct for L1leanrning, there are still a number of logical possibilities concerning its role in L2 learning.

1. No access hypothesis: UG is not involved in L2 acquisition, it based on age, and L2 learners have to resort to more general problem-solving skill.

2. Full access hypothesis: UG is accessed directly in L2 acquisition, and L1 and L2 acquisition are basically similar processes, the differences observed being due to the difference in cognitive maturity and in the needs of the learner.

3. Indirect access hypothesis: UG is not directly involved in L2 acquisition, but it is indirectly accessed via the L1. Therefore, there will be just one instantiation of UG which will be available to the L2 learner, with the parameters already fixed to the settings which apply in the L1.

4. Partial access hypothesis: some aspects of UG are still available and others not, for examples, principles might still available, but some parameter settings might not.

Furthermore, the Natural Order hypothesis is based on research findings which suggested that the acquisition of grammatical structures follows a 'natural order' which is predictable. For a given language, some grammatical structures tend to be acquired early while others late. This order seemed to be independent of the learners' age, L1 background, conditions of exposure, and although the agreement between individual acquirers was not always 100% in the studies, there were statistically significant similarities that reinforced the existence of a Natural Order of language acquisition. Krashen however points out that the implication of the natural order hypothesis should be based on the order found in the studies. In fact, he rejects grammatical sequencing when the goal is language acquisition.

Second language acquisition has more theories, acquisition order and developmental sequences: silent period, formulaic speech, natural order of acquisition, sequence of acquisition to formal instruction and error analysis. I do as the way I have learned. From my learning experience, error analysis is an interested acquisition in second language. Analysis did not seem to be borne out in practice. I was finding out in the classroom that constructions that were different in pairs of languages were not necessarily difficult, and that constructions that were similar in two languages were not necessarily easy either. Moreover, difficulty sometimes occurred in one direction in one direction but not the other .For example: the placement of unstressed object pronouns in English and Chinese differs: whereas English says I like them very much, Chinese says I very much like them. This was the origin of error analysis of mine. However, error encourages my language learning. In my opinion, developmental sequence is one of the most important acquisitions in second language.

To sum up, the evidences show that second language learners use creative strategies which are not dissimilar from those used by first language learners. It is true that children appear to acquire a second language faster than adults, but many adults have attained a high level of fluency in a foreign language. These studies indicate that attaining a working ability to communicate in a new language may actually be easier and more rapid for the adult than for the child. As an adult language learner, I believe that adults are not too old to learn a second language well and second language acquisition theories are helping me to improve my L2 knowledge.