I am confused about the usage of Present Perfect and Past Simple Tense. An example:
We are expecting normal results since the manufacturer maintained/has maintained good material quality in the past.
Which should be used here?
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You are worrying about the results 'at present'. And, you are confident that you'll get good results. Why? Because the track/record of the maker is fine. So, go for the third possibility:
We are expecting normal results since the manufacturer has been maintaining good material quality
in the past.
Because in other cases, it'd be the matter of the past and you are not sure about the quality at present. The manufacturer is dealing with you at present.