[chryuop]

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?