Stephen Krashen, Linguist & Educator


Dr. Stephen Krashen, Linguist & Educator
Dr. Stephen Krashen

Current second language acquisition theory owes a great debt to the pioneering work of Stephen Krashen, a linguist, educational researcher, and professor emeritus at University of Southern California.  When Krashen proposed his 5 hypotheses in the early 1980s, they offered a new and radically different view of language acquisition.   While subsequent research has uncovered both strengths and weaknesses of each of these hypotheses, I would suggest that Krashen’s five hypotheses outline the primary starting point or theoretical basis for second language acquisition today.  They are referred to in brief form as:

  • The Acquisition-Learning Hypothesis,
  • The Natural Order Hypothesis,
  • The Monitor Hypothesis,
  • The Input Hypothesis, and
  • The Affective Filter Hypothesis.

In these articles, our intention is to briefly explain and clarify each of these hypotheses.

The Acquisition-Learning Hypothesis

According to Krashen, “adults have two distinct and independent ways of developing competence in a second language.”  The first is “language acquisition,” similar to the process by which children develop ability in their first language.  This is not the same as “learning” a new language.  This distinction is vital!

Krashen has said, “language acquisition does not require extensive use of conscious grammatical rules, and does not require tedious drill,” so it is therefore more of a “subconscious” event.  Acquisition refers to responding naturally to what we hear, expressing our own thoughts and feelings without conscious reference to any rules.

The second way of developing language competence is called “language learning.” Personally, when I try to intentionally “learn” a language, I oftentimes get stuck with all the grammar and usage rules, as well as in a tedious process of repetitive drills designed to help solidify my use of those rules. That’s more of a teaching style, but it’s typical of the process of “language learning.”  We develop formal knowledge about a second language, how it works and how to talk about it.

Children acquire language skills quite naturally
Children around the world acquire language skills quite naturally

The key here is to recognize that the natural process of language acquisition is much different. Think back to when you were a young child.  You learned by hearing and mimicking language, not by first learning all of the grammar rules.  This is an example of “acquiring” a language. You were not able to speak in full sentences at first, nor did you have every word in your vocabulary right off the bat.  You learned naturally by listening and quite unconsciously acquired words and grammatical forms introduced to you in the course of life.

Later, in school, most of us had classes in which we learned to read, to write, and to express ourselves in correct, proper, grammatical English.  We learned the rules which explain “why” and “how” our language works. This may help solidify your success in communicating, but it is an essentially separate process from language acquisition.

The Natural Order Hypothesis

Studies have been done to try and determine if there is a natural order in persons begin “acquiring” various language structures, i.e. morphemes, syntax, etc.  The exciting discovery is that the acquisition of grammatical structures does indeed tend to follow a predictable order. In addition, what comes naturally for those learning their first language is very similar to (but not the same as) what comes naturally when learning a second language.

For example, as regards acquiring English as a second language, clear patterns emerge, as illustrated in the following chart from Krashen’s book.Natural order of second language acquisiton chart

Similar patterns also hold true for learning English as a first language.  Both have adding “ing” and making plurals by adding “s” early, followed by “irregular past” and “regular past”.  However, “auxiliary” and “copula” forms are acquired later when learning English first as opposed to second.

In other words, there is a fairly predictable order “in which mature, or well-formed structures emerge.”  Other research has shown there are also typical developmental errors and transitional forms which language acquirers pass through along the way to mature expression.

Of course, teachers would like a curriculum that helps students succeed in learning.  Unfortunately, it is not so simple as teaching more simple things first following the order of average mature acquisition.  In fact, Krashen himself cautions against this.  For example, Glossika courses – based on this research – are built using the most frequent syntactic structures found in language, rather than following a strict progression from simple to complex.  The intention is to allow you to acquire a large range of expression in as little time as possible.

The Monitor Hypothesis

The Monitor Hypothesis describes the relationship between language acquisition and language learning.  In particular, acquisition normally triggers our speech in a second language and is responsible for our fluency.  By contrast, conscious learning serves only a limited role as a Monitor to correct our speech.

It is suggested that persons acquiring a second language tend to employ the conscious rules of language learning only when three conditions are met.  The speaker needs to 1) know the appropriate rule, 2) be focused on correctness, and 3) have sufficient time to use the rule.  It’s difficult to focus on both expressing one’s ideas and employing correct form at the same time.  So, “even when ESL students write compositions with plenty of time and under instructions to be very ‘careful,’ the effect of Monitor use [is] surprisingly light.”

Of course, some people try to Monitor themselves all the time, generally resulting in hesitant speech and loss of fluency.  Other language learners under use their Monitor, depending instead on what feels right – even while acknowledging the importance of proper grammar.  According to Krashen, our goal in teaching is to produce optimal Monitor users, who use the Monitor “as a supplement to their acquired competence,” “when it is appropriate and when it does not interfere with communication.”

The Final Two Hypotheses

The final two hypotheses – the input hypothesis and the affective filter hypothesis – have the most practical application for both language acquisition and language teaching.  We will address these in Part 2.

In the meantime, please add your own questions and comments below, and I’ll make sure to respond as soon as possible.


  • Stephen D. Krashen, Principles and Practice in Second Language Acquisition, 1982 (first internet edition 2009), chapter 2 “Second Language Acquisition Theory.” All quotes reference this source.
  • Michael Campbell, “Second Language Acquisition vs Learning,” Glossika Blog September 3, 2015.


  1. Hey, I found this article on acquisition/learning very interesting. I resonate strongly with this because when I was 6 years old I moved to a whole different country which speaks a different language. I went from Mexico to Canada from Spanish to English. I learned English quickly as I learned it naturally to what I heard in school. I suppose this is called acquisition. In your opinion what is a faster way to learn, naturally, or through textbooks and such?

    1. Thanks for sharing your experience, Jose. I’ve spoken English all my life and am soon to move to Mexico; I hope my fluency improves quickly. To get to your question: Based on what I have been learning, it seems the natural process of language acquisition is significantly faster. What is exciting is that we are learning new ways to teach which enhance this process through intentional comprehensible input (see Part 2 of this article). We also recognize the role which more formal language study plays as a Monitor which — under the right conditions — helps us communicate in our new language more “correctly.”

  2. Interest is the best teacher, which is particularly true with second language learning. Language is a vehicle for information and tool for communication. If you are dying to know a particular information or keen to understand another person, second language learning will be tremendously rewarding. Otherwise, the learning process could be very boring: memorising grammar, memorising words without seeing the inner relationships between them and without seeing the purpose.

    Thanks for your post.

    1. Thanks for sharing. I agree that interest or motivation is very important! (See Part 2 of this article re the Affective Filter Hypothesis.) The “learning process” as you describe it can be very boring even for those very interested. Since the “acquisition process” focuses instead on understanding, it’s often much more rewarding.

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