Select the most appropriate segment to substitute the underlined words in the given sentence.
After he leave, we discussed his suggestion.

The correct answer is "he had left".

Step-by-Step Explanation:
1. Analyze the original sentence: "After he leave, we discussed his suggestion."
2. Identify the grammatical issue: The phrase "we discussed" is in the simple past tense, indicating that the discussion took place in the past. Therefore, the action of him leaving, which happened before the discussion, must also be in the past. The phrase "he leave" is grammatically incorrect because "leave" is in the base form and does not agree with the singular pronoun "he" in the past context.
3. Determine the correct tense sequence: When two actions occur in the past, the action that happened first is expressed in the past perfect tense (had + past participle), while the subsequent action is expressed in the simple past tense. Here, the action of leaving happened before the action of discussing.
4. Evaluate the options:
- "he will be leaving": Future continuous tense. Incorrect because the context is set in the past.
- "he has left": Present perfect tense. Incorrect because it connects a past action to the present, whereas both actions in the sentence are fully completed in the past.
- "he had left": Past perfect tense. Correct because it properly shows that the leaving occurred before the past discussion.
- "he was leaving": Past continuous tense. Incorrect because it suggests the leaving was in progress during the discussion, which contradicts the chronological marker "After".