Choose the correct form of verb: if I — earlier, I would have caught the train.

This question tests knowledge of conditional sentences, specifically the third conditional.

• Foundational Fact: The third conditional is used to talk about hypothetical situations in the past and their hypothetical results. Its structure is: "If + past perfect (had + past participle), would have + past participle." The main clause "I would have caught the train" clearly indicates a third conditional structure.
• Correct Answer (C): "If I had left earlier, I would have caught the train." "Had left" is the past perfect tense, which correctly completes the "if" clause of a third conditional sentence. This expresses a past condition that did not happen and its unrealized past result.
• Why Distractors are Wrong:
  • (A) "Leave" is present tense, used in the first conditional (e.g., "If I leave, I will catch").
  • (B) "Left" is simple past tense, used in the second conditional (e.g., "If I left, I would catch").
  • (D) "Would leave" is incorrect in the "if" clause of any standard conditional structure. The auxiliary "would" is typically reserved for the main clause.