The word which best completes the given sentence is
When a person is speaking grammar and wants to speak or write it correctly according to the rules of grammar, then he would have to obey the rules of concord.
This rules of concord are used to make sure that the subject and verb agree in both number, tense, etc and we can see that there is the narration about a future event and the word "Will" is the answer.
Read more about sentence structures here:
https://brainly.com/question/11554797
