1. J9 Editing & Proofreading Service Common Grammatical Errors
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10. (continued) If we change that to which then we have to insert commas: ‘ The computer, which Jane was using at the time, was too slow for the work she was doing.’ The sentence now means: ‘The computer was too slow for the work Jane was doing.’ It’s not entirely incorrect but it is unclear. We don’t know whether Jane always works on this computer or not. Therefore, the words that Jane was using at the time are important to the sentence and that must be used. To use which means we can omit them from the sentence and this makes the sentence unclear.