Explicit grammar instruction, though never entirely absent from the second language syllabus and classroom, was for long periods out of fashion in second language research circles.
On one hand, behaviorist approaches eschewed explicit instruction, preferring drilling without analysis in order to instill good habits. Chomskyan cognitivists, on the other hand, relied on innate language learning mechanisms to convert input to competence. For Krashen, this meant little or no role for grammar instruction in the classroom.
However, classroom research by Swain and others shows the limitations of exclusively communicative approaches in immersion education. Long suggested a return to explicit grammar instruction, but embedded in a communicative framework.
Focus on form ... overtly draws students' attention to linguistic elements as they arise incidentally in lessons whose overriding focus is on meaning or communication.
Doughty points out that focus on form contrasts both with traditional grammar-based instruction which excludes meaningful communication, and communicative approaches which prohibit grammatical instruction.
Input enhancement, input flooding, and auditory recasts involve implicitly focusing learners' attention on particular elements, leading to cognitive comparison and thus interlanguage development.
Read more (Long 1997).