What is the difference between Dishwashers were introduced in 1978 and Dishwashers had been introduced in 1978
Thank you in advance.
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"Were introduced in" gives a precise year. They were not introduced before that date.
"Had been introduced in" is more vague. It can mean a precise year of introduction. It can also mean that they were introduced in that year or earlier, although that would more commonly be written as "had been introduced by".
The past perfect (whether simple, passive, continuous, going to, or anything else) always means that the speaker has chosen to view the event from a more recent point in the past. It is nearly always a free choice for the speaker - it does not depend on the objective sequence of events. English speakers often don't bother with it when the temporal relationships are clear.
Dishwashers had been introduced in 1978
makes perfect sense if you are already talking about something at a time in the past later than 1978. It is incoherent if you are not.
Dishwashers were introduced in 1978.
This is a very simple statement denoting the fact that dishwashers have been around since 1978 and no earlier.
Dishwashers had been introduced in 1978.
This statement is sort of weird. People would expect you to say more about the topic in continuation if you use past perfect tense. Past perfect tense is used to speak of events which happened in the past, one after the other (series of events). Also, it would seem like dishwashers were introduced in 1978, and might be after that they were never ever used or demolished altogether, which is not the case we are looking for, unless you are writing a fictional story that is.