The boy sitting next to me on the plane was nervous because he … before.

• has not flown
• had not flown
• did not fly

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In this sentence, we need to choose the correct past tense form to complete the phrase "because he … before." Let's break down the options:

1. has not flown: This form is the present perfect tense. It is used to describe an action that happened at an unspecified time before now. However, in this sentence, we are dealing with a past situation, so the present perfect tense is not appropriate here.
2. had not flown: This is the past perfect tense. It is used to indicate that an action was completed before another action in the past. In this sentence, the boy was nervous because he had not flown before the action of sitting next to you on the plane. This makes the past perfect tense the correct choice.
3. did not fly: This is the simple past tense. It describes an action that was completed in the past. However, it doesn't fit well in this context because we are talking about an experience (or lack thereof) prior to the moment of sitting on the plane, not a specific action that happened.

Therefore, the correct answer is "had not flown." The completed sentence is: "The boy sitting next to me on the plane was nervous because he had not flown before."