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The song [eX
Dream](http://tw.myblog.yahoo.com/cathy910732/article?mid=1128&prev=1129&next=382&page=1)
by Myuji (which appears [in an anime
OP](http://en.wikipedia.org/wiki/List_of_X_episodes)) contains the sentence:

> [夢]{ゆめ}は夢でしかない

The general point of the sentence is fairly clear, meaning "a dream is just a
dream". But what is the best way to think of the でしかない construction?

At first I saw it as 夢は夢で, as in, "a dream is a dream and...", but now I'm
wondering how correct this is or if it's correct at all.

Is it better to look at it as a separation of the parts of である? Does this
happen in many other cases, where you use a qualified で followed by ある?
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question
grammar particle-で copula
Grammatical justification for でしかない
If you were to say 夢は夢しかない then it would have a meaning of something like
"dreams have nothing but dreams." It's the simple ～は～が construction you learn
in Japanese 101 to describe a particular feature of a subject. This is not a
copula.

As you mention, you should be looking at it in terms of である. If you take out
the しか you'll have the normal copula 夢は夢ではない, which you will recognize is that
full form of "じゃない." So this way of sandwiching しか into it allows you to get
the "only" meaning of しか while preserving the copula である.

As for other cases, I"m not sure if you mean other cases of でしか or other words
with である. For でしかない you can expect to see it wherever the meaning is
appropriate as it's not a particularly exotic expression. For other
combinations, you can compare it to でもある, which is how you say that [something
is _also_ something else](https://japanese.stackexchange.com/q/1905/78)
(その人は研究者です。先生でもある。）.
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answer
Let's start with something common:

> 夢は夢だ。 'Dreams are dreams.'

Let's negate it (using ではない instead of its contracted form じゃない):

> 夢は夢ではない。 'Dreams are not dreams.'

は is a 係助詞{かかりじょし} ("binding particle"). Any 係助詞 fits in this spot. しか is also
a 係助詞:

> 夢は夢でしかない。'Dreams are nothing but dreams.'

* * *

The "modern" grammatical analysis of this stuff is that で is the 連用形{れんようけい}
("continuative form") of the copula だ which results in ではない and でしかない falling
nicely into this general rule:

> (連用形 of something) + (optional 係助詞) + (ない or ある)

Some other instances of this rule are `すごく（は）ない`、`あつく（も）ない`、`バカで（も）ある`。
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answer