the sentence, a subject-auxiliary inversion, and in some cases the insertion of do in the right place. Rules can also be easy and difficult due to their semantic properties. The English article system is easy to describe formally--one simply inserts the or a or sometimes nothing before the noun. But its semantics are very difficult to describe (see, for example, Hawkins, 1978).

To summarize thus far, Monitor use results in the rise in rank of items that are "late-acquired" in the natural order, items that the performer has learned but has not acquired.³ Only certain items can rise in rank, however, When Monitor use is heavy, this rise in rank is enough to disturb the natural order. (As discussed in Chapter IV, it is possible to see small changes in certain late-acquired morphemes that are not enough to disturb the natural order; this may be termed light Monitor use. See especially Note 5, Chapter IV.

As we shall see in Chapter IV, it is not easy to encourage noticeable Monitor use. Experimentation has shown that anything less than a real grammar test will not bring out the conscious grammar in any force. Keyfetz (1978) found natural orders for both oral and written versions on the SLOPE test, showing that simply using the written modality is not enough to cause an unnatural order. Houck, Robertson and Krashen (1978a) had adult subjects (university level international students) correct their own written output, and still found a natural order for the corrected version. Krashen, Butler, Birnbaum, and Robertson (1978) found that even when ESL students write compositions with plenty of time and under instructions to be very "careful", the effect of Monitor use was surprisingly light. The best hypothesis now is that for most people, even university students, it takes a real discrete-point grammar-type test to meet all three conditions for Monitor use and encourage significant use of the conscious grammar.

(a) Individual variation in Monitor use

Some of the individual variation we see in adult second language acquisition and performance can be accounted for in terms of differential use of the conscious Monitor. Studies of case histories suggest that there may be three basic types of performer (Krashen, 1978; Stafford and Covitt, 1978; Kounin and Krashen, 1978).

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