Which is correct - He was in the same class as John and I or is it the same class as John and me?
you should say :

He was in the same class as John and I were OR He was in the same class as John and me

He was in the same class as John and I were.


He was in the same class as John and me.


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I wouldn't say "as John and I were."

I would perhaps say "the same class that John and I were in." I'd be more likely to say "the same class as John and me."
Hi Grammar Geek

Is "that" optional here or not?

Thank you in advance
I do think it's optional, but I would probably use it. (But I think that "as" there is wrong.)
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I disagree. The grammatically correct form is "as John and I (were)" where "were", "were in", etc. are optional continuations. The answer "same class as John and me" is grammatically incorrect, though tolerated informally. It is similar to: "Who wants ice cream?" "I.", which is the correct answer and is short for the (optional) "I do". However, almost everyone will say "Me."; not only is the latter widely tolerated, but it is accepted to the point where "I" would usually be considered pretentious. Nevertheless, standard grammar considers "I", "same as I", "same as John and I", "greater than I", "better than I", etc. as formally correct, and "me" as incorrect but informally tolerated in these contexts. None of this should be confused with the personal pronoun as object complement, which frequently is erroneously hypercorrected today. So you should say, "She gave it to John and me", ALWAYS, formally or informally, but people who try to sound more educated than they actually are tend to "hypercorrect" to "... gave it to John and I", which is awful. In the first case under discussion, "same as", "better than", etc. is followed by a second grammatical subject, not an object; therein lies the tendency to err. A good test is if you can add "am" or "were", which shows that it is really a subject, not an object. But this is a test, not a necessary condition for employing the pronoun. In other words, you don't say "I" only if you actually SAY "were"; it is sufficient to have to say I if you merely COULD say "were" (whether or not you actually do so). That is why the previous reply to your question is incorrect.