Complex sentences are written because the writer feels a need to give more detailed or more accurate information when expressing an idea. This automatically needs the use of different grammatical structures because that is why they exist. The present perfect exist to help the writer describe a situation that began in the past and continues to the present. Let's look at a simple context sentence:
Smoking is a serious problem in our society today.(simple sentence)
But this problem started a long time ago. So we could include the adverbial clause of time and has been for a very long time in our sentence to express our idea more accurately:
Smoking is today, and has been for many years, a serious problem in our society.(using the present perfect)
This sentence is not only more informative, but is technically a superior sentence and so would help get better marks in an exam.
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