Question about で
 

A couple of days ago I learned a new use (new to me) for the particle で. In another forum someone asked why で　was used in the phrase 皆で食べられるようにたくさん作った。。。We all were reasoning as passive form wondering why the で was there. Till a native speaker came on and told us it was a potential form and even though his English wouldn't let us explain why, we figure out that the "agent" in potential forms is marked by で.

Now, on language 8 I corrected a Japanese guy who wrote in my language and then he translated it also in Japanese. Amongst others there was this phrase: 私は夫婦でお店を経営しています which she translated in my language as "I have a shop withmy husband". Is this で related to the one in the potential form, such as the one performing the action? But shouldn't it be が in this case? I mean, in the potential form the が can mark the object thus you need to mark who is performing the action (日本語が皆で話せる...lol I wish). In a regular phrase I don't see the need to have で for the one performing the action...

Dang it, the more I try to explain it the more confusing it comes out. But I guess it comes out confusing coz I have a confusion in my head about it LOL

It's just a wild guess, but 夫婦 is sort of like an adjective/noun/state-of-being thingy (meaning "husband and wife"), not a person, so you wouldn't use と there.

I wonder if 皆 serves a similar function in the first sentence. Namely, that 皆 is not a person identifier, but something like a "state of being." "As a group of everyone" rather than "with everyone."

Regardless, think of the second one not as "with my husband" but "as husband and wife" or "in the husband-and-wife state" instead, and you won't run afoul of a と／で confusion.

This で describes who is/are the one(s) performing the action that is being discussed. Trust me, it's in heavy use.

皆で食べられるようにたくさん作った

Hope you asked yourself two questions in understanding this sentence.

NOTE: The action that I speak of is the "eating" in this sentence.
NOTE: The verb being in its potential form has nothing to do with the use of the で.

1. Who made the food? = What is the subject of this sentence?
2. Who will eat it.

Answers:

1. Only the context will tell.
2. Everyone.

"(Someone) made a lot of (food) so that everyone could eat it."
__________

昨日日本語のクラスで花見に行った。

家族でホテルを経営しています。

ふたりで幸せになろうね。

ドンとジョンは兄弟でハンバーガー店を始めた。

私たちが家族でゆっくりできる旅館を田中さんが探して くれた。

家族５人で住めるようなアパートはありますか。

毎年一度会社で旅行に行きます。

ディズニーランドにはグループで来る人が多い。

EDIT: Forgot to mention the best example! This should explain things. 　

ひとりで

ひとりで行った
３人で行った
家族で行った
会社で行った
みんなで行った

I thought that the action performed with で was marked only in the potential form...and yes 一人で was the comparing word I had pulled out in the other forum.
However if the is used also in non potential forms, now I have another questions. I take the first example you wrote...
昨日日本語のクラスで花見に行った and 日本語のクラスが昨日花見に行った...where is the difference? Can が and で be exchanged freely?

The difference is that the former is the natural-sounding sentence and the latter is something only Japanese-learners would say.

Wow! This is made so much easier to understand because you have taken the time to give example sentences. :)
I know I wasn't the one who asked the question, but this thread has definitely helped me. Thanks Masaegu!

Thanks, jesselt!

The important thing is that the 「～～で」 part is NOT the subject of the sentence. It simply adds the information "in a group of ( )".

This is why the two sentences
日本語のクラスで花見に行った and
日本語のクラスが花見に行った
sound and feel very different from each other to the native speaker.

Does the second sound non-native, or is it that the difference is more like:
"I went cherry blossom viewing in a group with my class" versus
"The class went cherry blossom viewing"?

I can sort of see how a language could treat クラス as a container rather than a noun that can perform actions, and thus "クラスが行く" could sound weird. But before I make this assumption, I'd like some confirmation.

As a sort of parallel, in American English, teeechnically we are taught in school that inanimate objects cannot possess objects, so "the car's bumper" is wrong, while "the bumper of the car" is correct. But I said "technically" for a reason, since all native speakers will say the first anyway since it's shorter.

The only natural situation that I can think of where the second sentence might be used by a native speaker would be as the answer to the question: "What class went on a hanami?"

If I have to think about a situation where something can be said, that's how often we would actually say it.

This is just like how we would say カイルが窓を割った. We would say it only as the answer to the question: "Who broke the window?"　In other situations, we would surely use は.　

If you are among the group that went on a hanami, you would say 日本語のクラスで花見に行った, not the other one. Having gone to school for 16 years in Japan, I just KNOW it. I've been on several dozen field trips and not once has anyone used が in talking about them!

Thank you. Confirms and strengthens my instinct.