“Though a skill in its own right, grammar can also be regarded as a necessary “master” skill that enables competence to develop in the areas of listening, speaking, reading, and writing. When grammar is incorrect or misunderstood in any of these areas, communication may be disrupted, as the following examples illustrate.
Listening. A teacher who says, “Please bring me the books,” only to have a student bring her just one book because the student did not hear the plural -s or understand what it means. In this case, a better understanding of the underlying grammar would have improved the student’s listening ability.
Speaking. An applicant is asked, “How long have you been working at your current job?” The applicant replies, “I worked there for two years.” The interviewer wonders: Is the applicant still working there or not? In this situation, knowledge of the present perfect would have enabled the student to reply more accurately.
Reading. Trying to follow the directions for assembling a bookcase, a student reads Slide the bookcase close to the wall after tightening all of the pieces. Not knowing that after signals the first of the two actions in the sentence, the student performs the actions in reverse order.
Writing. An automotive tech student writes I check the brakes. The supervisor is confused: Did the student already check the brakes? Is he going to check the brakes? If the student had written checked on the report, there would have been no ambiguity.
Taken from: Grammar Matters: pages 2-3 Teaching Grammar in Adult ESL Programs by K. Lynn Savage with Gretchen Bitterlin and Donna Price. Cambridge University Press New York, NY 2010.