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8.6 present simple, present perfect and simple past reference to the literature 1.
Past simple to refer to the literature when you give the date within the main sentence (i.e. not just in parentheses.
2.
Present perfect to give past-to-present background information. The
present simple is possible here, but is much less commonly used.
3.
Present perfect if the method, technique, procedure etc is the subject of the verb and there is no time reference,
past simple if the author is the subject of the verb. In such cases there mayor may not be a time indication.
4.
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