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My friend told me that they are both grammatically correct and refer to the same meaning.

1) "They had all known that Tom had a girlfriend before I told them"
2) "They all knew that Tommy had a girlfriend before I had told them"

My question is that the Past Perfect tense should only be used to refer to an action which is happened before another action at the past. In another word, "Past Perfect Tense" happened first then "Past Tense" happened. In the situation above, they all knew the news before I told them. Therefore, I reckon only the second one is correct.

Please correct me if I have mistaken any thing here.

Cheers
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ConscientiousStudent 1) "They had all known that Tom had a girlfriend before I told them"
2) "They all knew that Tommy had a girlfriend before I had told them"
Not only conscientious but prescient as well ! Or have you been reading my posts??

Many words in all categories carry a contextual or semantic meaning that relates to time, as does "before" in your sentences. These meanings can complement the functions of certain verb tenses, or even replace them. When used incorrectly, these "time" words may conflict with the verb tenses.

I'm afraid your reckoning has carried you in the wrong direction. Only the first one is correct.

In your second example, the "telling" is in the past perfect, and the "knowing" is in the simple past. That means the telling came first, which I don't believe is your intention.

Number two says:
"He threw the ball before I had hit it." What you want is, "He had thrown the ball before I hit it." OR "He threw the ball before I hit it."

You can place both verbs in the simple past, and let the preposition "before" do its job. Such an arrangement will carry the same meaning as number one. "They all knew that Tommy had a girlfriend before I told them."
Comments  
hi there,
while gramatically speaking one might be more correct than the other, i think we say the first when adding emphasis to the fact that everybody already knew....at least I think this is the case in oral english.
perhaps someone disagrees?
BR
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 Avangi's reply was promoted to an answer.
Hi Guys,

Thank you for your reply. Avangi , you are very right. It was a typo, I got it another way round. Ha ha ha.....stupid me. Now I got better grasp on those tenses.

Cheers