Why should we teach grammar to L2 learners?

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Grammar shouldn't be the first thing we teach in L2 classes, but it needs to be taught at some point. Grammar is all about patterns. Some students will pick up on these patterns without explicit instruction, but most won't, especially if the patterns are very different from those in the students' L1. Explaining these patterns to the students makes it easier for them to remember what morphemes should be used where and when.