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Question

Difficulty: MediumVerb Tense, Aspect, and Mood

To ensure the passage conforms to the conventions of Standard English, which form of the verb 'discover' should be written in the blank?

Answer:Since the launch of the Kepler Space Telescope in 2009, astronomers 【have discovered】 thousands of candidate exoplanets, transforming our understanding of stellar systems in our galaxy. Before this pivotal mission, researchers could only detect massive, gas-giant planets orbiting extremely close to their parent stars; however, Kepler's high-precision photometry enabled the detection of smaller, rocky worlds that orbit within habitable zones.

Answer

have discovered
The correct answer is 'have discovered'. The sentence begins with the phrase 'Since the launch of the Kepler Space Telescope in 2009', which signals a time period that started in the past and continues to the present. This ongoing timeline requires the present perfect tense. Because the subject of the verb is the plural noun 'astronomers', the plural form 'have discovered' must be used.

Step-by-Step Solution

1
Identify the temporal markers in the sentence containing the blank.
The prepositional phrase 'Since the launch... in 2009' establishes a timeframe starting in the past and continuing up to the present.
Temporal markers dictate the correct aspect and tense of the verb in context.
2
Determine the correct verb tense and aspect based on the timeframe.
The present perfect tense is required to show an action that began in the past and has relevance or continuation in the present.
Simple past would isolate the action entirely in the past, whereas the context shows an ongoing period of discovery from 2009 to the present.
3
Identify the subject and match it in number with the verb.
The plural subject 'astronomers' requires the plural auxiliary verb 'have', resulting in 'have discovered'.
Verbs must agree with their subjects in number as well as tense.

Key Concept

Using the present perfect tense to express actions that started in the past and continue into the present, particularly when introduced by the word 'since'.
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Topics

Verb Tense, Aspect, and Mood

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