The subjunctive construction is difficult to learn as explicit knowledge, because it is formally and functionally very complex, and because it specifies the peripheral functions of the relevant
forms. This might explain why task performance did not improve in proportion to the increase of practice frequency, although there existed a large room for improvement. At a certain point, more practice stopped resulting in more restructuring of the explicit knowledge system.
However, how come that the group who practiced twice managed to develop its explicit knowledge to the same extent as the group that practiced six times? One explanation was that the subjunctive construction had been explicitly taught to the participants as rules, so when they were engaged in explicit learning during practice and were doing the correction task, recall of these rules was easily triggered, which could facilitate the learning and use of this explicit knowledge. Another likely explanation consisted in the conscious awareness this construction tended to induce. It has high perceptual salience and high communicative value, so it tends to
draw learner attention. Besides, according to Bley-Vroman (2002) , many things that are encountered very rarely may be noticed, processed and consequently learned, because they will strike the learner as salient. Since this construction is low in frequency in the input learners are usually exposed to ( see R. Ellis, 2006), it was likely to be noticed when it was encountered, even by learners with fewer practice opportunities, and such noticing made explicit learning possible.
On the other hand, learners' performance in the speaking task indicated that development of implicit knowledge largely reflected practice frequency, with more practice leading to more learning.
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