Introduction

There has been a long-term debate concerning whether there are two functionally separate systems in human learning. An influential theory has been proposed in Krashen and Reber arguing that an unconsciously acquired knowledge was developed independently of a consciously learned one. In the past three decades, a substantial number of empirical studies have been conducted to address the effectiveness of various types of instructional treatments in the acquisition of a foreign or second language (L2). An interesting finding is that the choice of target forms may influence the effectiveness of L2 instruction due to the fact that different linguistic features pose different requirements on acquisition and hence interact with different learning conditions. The effect of rule awareness in acquisition is still controversial in terms of its due contribution to learning. Despite the general agreement on a beneficial role played by attention in language learning, different opinions exist regarding the amount and type of attention/awareness needed for learning to take place.