owe its source to Piaget's Formal Operations stage. Affective changes that occur around puberty, some related to Formal Operations, affect language acquisition. The chapter concludes with a re-definition of the "good language learner", now defined as someone who is first and foremost an acquirer, and who may also be an "optimal Monitor user".

Chapter 2 originally appeared in Diller (1980).

Formal and Informal Linguistic Environments

Chapter 3 is a revised version of a paper that appeared in the TESOL Quarterly in 1976 (see Krashen, 1976a). It shows how the acquisition-learning distinction helps to solve a puzzle in the second language acquisition research literature: several studies apparently show that formal learning environments are best for attaining second language proficiency, while other studies appear to show that informal environments are superior. In this section, it is argued that informal environments, when they promote real language use (communication) are conducive to acquisition, while the formal environment has the potential for encouraging both acquisition and learning.

This chapter, then, begins the discussion of the potential of the second language classroom for language acquisition, a discussion that is continued in later sections (Chapters 8 and 9).

The Domain of the Conscious Grammar: The Morpheme Studies

Chapter 4 reviews research pertaining to acquisition or difficulty order of certain structures, that is, which structures adult second language acquirers tend to acquire early and which they tend to acquire late.

The value of these studies is considerable. They provide more information than merely showing us the actual order of acquisition. They also show us when performers are using conscious grammar and when they are not. We have hypothesized that when conditions for "Monitor-free" performance are met, when performers are focused on communication and not form, adult errors in English as a second language (for grammatical morphemes in obligatory occasions¹) are quite similar to errors made by children acquiring English as a second

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