Krashen claimed that explicit learning of grammar rules does not lead to the acquisition of linguistic competence; acquisition occurs only subconsciously when the learner is exposed to comprehensible input, or samples of the languages which he or she can comprehend (Input Hypothesis).
DeKeyser calls this the non-interface position: there is no link between explicitly learned rules and implicitly acquired linguistic competence.
Hence the importance of the implicit/explicit learning distinction: is all acquisition implicit, or does explicit knowledge play a role?
How are implicit and explicit learning defined?
Implicit learning - learning without awareness (French & Cleeremans, 2002) - is contrasted with explicit learning, which generally focuses on rules, either deductively, in traditional grammar-based instruction, or inductively, when learners are invited to infer rules from input. The terms incidental versus intentional learning cover approximately the same contrast.
Traditional information processing models assume that explicit learning (declarative knowledge) gradually becomes implicit (procedural) knowledge through practice (automatisation) leading to restructuring. Recent emergentist models make the reverse assumption: some complex skills are acquired implicitly, and learners cannot verbalise (explicitly describe) these processes.
Read more: About emergentism