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> 昨日はひどい雨 **で** 雷も落ちて、大変でした。

I would translate it like this:

"Concerning yesterday, there being terrible rain and thunder dropping it was
terrible."

In this case I assumed that で represents the connective form of です/だ.
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2017-07-11T12:46:49.730
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2017-07-12T06:58:03.190
2017-07-12T06:58:03.190
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question
grammar
Is this で a particle or the connective form of copula? Or something else?
You're right. Literally, "Yesterday, it was terrible rain and there were claps
of thunder and it was awful." Since the basic sentence is 昨日は大変でした, you could
make "yesterday" the subject of "was awful" in an English translation:
"Yesterday was awful - there was terrible rain and claps of thunder".
CC BY-SA 3.0
2017-07-11T13:06:32.757
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2017-07-12T06:56:23.610
2017-07-12T06:56:23.610
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answer