1) Grammar is the system and structure of a language that evolves over time based on how people use language, rather than fixed rules. It reflects how a language is used at a particular point in time.
2) Knowing grammar helps people effectively monitor and communicate the meaning and effectiveness of language use. However, usage is more important than fixed rules.
3) It is important for students to use correct grammar when writing or speaking to fully express their message to others and meet educational standards, though languages naturally change over time. Teaching grammar needs to consider learners' backgrounds and class objectives.