Choose the correct form of verb: If I _____ earlier, I would have caught the bus.

Had left is the correct form of the verb to complete the sentence "If I _____ earlier, I would have caught the bus." This construction is an example of a third conditional sentence, which is used to express a hypothetical situation in the past that did not happen. The third conditional typically follows the structure "if + past perfect, would have + past participle." In this case, "had left" fits perfectly into this structure, indicating that the speaker did not leave earlier, which consequently resulted in missing the bus.

Understanding the use of the past perfect tense is crucial for competitive exam preparation. It highlights actions that were completed before a certain point in the past. The phrase "had left" demonstrates that the action of leaving was supposed to occur before the moment of missing the bus, emphasizing the relationship between the two events. The other options—leave, left, and have left—do not fit the grammatical requirements of the third conditional, as they either represent the wrong tense or do not convey the necessary hypothetical nature of the situation.

When preparing for exams, it is essential to recognize the patterns in conditional sentences, particularly in identifying the correct forms of verbs. These types of questions frequently appear in competitive exams, so mastering them can significantly enhance your performance. The ability to choose the correct form, such as in this case with "had left," not only demonstrates grammatical proficiency but also showcases an understanding of the nuances of English language structures.