To complete the sentence "We _____ obey our teachers," we need to choose the correct modal verb. Let's analyze each option:
A: have to - This modal verb expresses obligation or necessity. It is commonly used when there is a rule or requirement that must be followed. For example, "We have to submit our assignments on time." However, in the given sentence, "have to" does not convey the appropriate meaning.
B: must - This modal verb also expresses obligation or necessity. It is often used to emphasize the importance of following a rule or fulfilling a duty. For example, "We must respect our elders." In the given sentence, "must" fits well and conveys the intended meaning of a moral or social obligation to obey teachers.
C: should - This modal verb expresses recommendation or advice. It suggests that something is the right thing to do or the preferred course of action. For example, "We should take care of our environment." While it is important to listen to our teachers, "should" does not express the same level of obligation as "must" in the given sentence.
D: has to - This modal verb is incorrect because it is in the wrong form. "Has to" is used for the third-person singular subject (he, she, it), while the sentence refers to the first-person plural subject "we."
Therefore, the correct answer is B: must. The completed sentence is "We must obey our teachers."