Teaching grammar has always been discussed whether we should teach grammar to language learners. Krashen (1982) states:

Language acquisition does not require extensive use of conscious grammatical rules, and does not require tedious drill.

in 1972 Chomsky states that language learning is innate and he defines:

All children share the same internal constraints which characterise narrowly the grammar they are going to construct.

The theory of Universal Grammar was suggested that all children were born with the ability to acquire and understand the grammar without knowing the rules. In Universal Grammar two concepts have been central of importance: 1) linguistic competence and 2) linguistic performance.

There are discussions whether grammar rules should be taught in ESL In classrooms, but the best way for learners to acquire the rules is through exposure to the language. Acquisition will occur quickly. It should be introduced in a context rather than giving the rules on the board and ask the learners to copy it in their book.

Further readings:

Krashen, S (1982) Principles and Practice in Second Language Acquisition.

Universal Grammar