GMAT Sentence Correction (SC) | Parallelism -- query

Sentence1:I STUDY hard, BUT I TAKE breaks.

Comma is used in the above sentence as separator: two main clauses are joined by comma + coordinating conjunction (BUT).

The sentence without comma can be written without repeating the subject as the subject of both verbs is "I".

Sentence 2:I STUDY hard BUTTAKE breaks.

Sentence elements are perfectly parallel in bothsentence.

Wrong construction: Use of comma + coordinating conjunction (BUT) without subject clause
[color=#ff0000] TUDY
...

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