What's the grammar structure of ここしかない in this sentence?

Question

安全な所はここしかない

I think there may be 2 explanations of しか in this sentence:

• しか replaced the case particle が. But if we transform it back and change the negation to the positive form: 安全な所はここだけがある.
  I added the だけ to imply the meaning of "only". But this sentence sounds a little unnatural, so I don't think it's the correct explanation.
• しかない is transformed form である: 安全な所はここだけである.
  It sounds natural and has a similar meaning to the original sentence. But to use しか for である, shouldn't it be 安全な所はここでしかない?

These 2 explanations correspond to ここがある and ここである respectively. But which is correct? The question maybe equal to this: Is the ない in the original sentence 非存在を表す形容詞 or 否定の助動詞?

Answer 1

First, Xでしかない and Xしかない have different meanings.

Xでしかない = nothing other than X; nothing more than X; merely X

• それは夢である。That's a dream.
  それは夢でしかない。That's nothing more than a dream.
  それは夢しかない。×

• 彼は学生である。He is a student.
  彼は学生でしかない。He is merely a student.
  彼は学生しかない。×

Xしかない = X is the only option; there is nothing but X; there is only X

• 行くなら候補は日曜日である。If we go, the option is Sunday.
  行くなら候補は日曜日でしかない。If we go, the option is nothing but Sunday.
  行くなら候補は日曜日がある。If we go, there is an option of (going on) Sunday.
  行くなら候補は日曜日しかない。If we go, the only option is Sunday.

• 正しい答えはAである。The correct answer is A.
  正しい答えはAでしかない。(You may be thinking B is correct but) the correct answer can be nothing but A.
  正しい答えはAがある。(You may be thinking there is no answer but) there is a correct answer, A.
  正しい答えはAしかない。(You may be thinking there are multiple correct answers but) the only correct answer is A.

• 安全な場所はここである。The safe place is here.
  安全な場所はここでしかない。The safe place is here (and not somewhere else).
  ^{(The focus is "which place is considered safe?")}
  安全な場所はここがある。As for safe places, we do have this place.
  安全な場所はここしかない。The only safe place is here.
  ^{(The focus is "how many safe places are there?")}

• 10円玉がある。There are 10-yen coins.
  10円玉しかない。There are only 10-yen coins.

Semantically, the Xしかない pattern denies the existence of other options, so it's ない for 存在, and that's why で must not be used.

But that does not mean we commonly say 候補は日曜日がある, 安全な場所はここがある and so on. These are natural only when you need to strongly emphasize the existence of X, as shown above. Normally, である can be seen as the natural "opposite" of those しかない sentences.

Also note that しかない is over 100 times more common than だけがある. There are some rare contexts where だけがある could sound natural, but let's not use it as the base of our discussion here.

Answer 2

This question seems to be based on the assumption that every negative sentence has to have an affirmative counterpart, but I don’t think it’s a valid assumption. It could be in a way related to this question, or more precisely, what Karl Knechtel said in the last paragraph of their answer. For me, the core part of the sentence is 安全な場所はない. It talks about the non-existence of something. I don’t see why it should be assumed to be convertible to a sentence that talks about the existence of something. Non-existence is non-existence. The core sentence might as well contain a phrase like ここ以外に, and such a phrase would be straightforwardly interpreted as modifying the predicate ない.

安全な場所はここ以外にない。

Then why can’t we see ここしか as playing a similar adverbial role in 安全な場所はここしかない?

It’s also kind of similar to how expressions of quantity work.

安全な場所は３つある。

By converting this into a negative sentence with しか, we get:

安全な場所は３つしかない。

This is different from 安全な所はここしかない only in that ３つ is a quantity and ここ is not. Both are exceptions to the general non-existence. Safe places don’t exist but, exceptionally, there are three places that are safe, or only this place is safe. I don’t see why these two require different explanations. I think we could simply accept that a negative sentence with しか allows a non-quantity expression in the same position where other sentences allow a quantity expression, unmarked by a case particle.

Incidentally, 安全な所はここしかない can also be understood as saying the same thing as 安全な場所はここにしかない, which is the flip side of 安全な場所はここだけにある. In this case, しか can be seen as appended to a に-phrase that denotes a place where something exists (or not).

The flip side of 安全な場所はここだけである would be 安全な場所はここでしかない, but it means a different thing. ここ is a predicate in this case. Few people would interpret 安全な場所はここしかない this way.

In any case, the subject of the sentence is 安全な場所 and not ここ. It’s wrong to compare it with any sentence where ここ is marked with が.

Answer 3

I think about しか as a noun-ish thing meaning "more than" or "other than". (This is wrong, check aguijonazo's answer)

So,

安全な所はここしか(が)ない。

しか would be the subject.

でしか would mean something like "more than a certain state of being" (check the examples given by "naruto"). Grammatically, で is the て form of the copula as in 無事でよかった。

This is congruent with the other answer but maybe not grammatically accurate.

I don't know why が is omitted. But this also happens when しか is used as a direct object 「肉しか(を)食べてない」 giving me the idea of noun-ish thing I talked earlier. (Again check aguijonazo's answer for an explanation that accounts for these problems)

I don't think しか transforms in any of the ways you described. Problems arouse when you assume that some transformation has to be taking place.

Now if you want to take the sentence:

正しい答えはAである。

And make use of しか, then you have to consider that this sentence uses the copula である. It is saying literally that "correct answer is A". However if you simply replace ある with しかない

正しい答えはAでしかない

"Correct answer being more than A does not exist".

In the first one the verb is "to be" (copula である) and in the second is "to exist" (ない negative of ある, not the copula である as copulas cannot be negated, think about です or だ). From this grammatical distinction, connotations follow as "naruto" described them.

Just for completion:

正しい答えはAがある

"Speaking of correct answer, A exists" (remember explicit が puts emphasis in the subject and see that the verb is "to exist")

正しい答えはAしかない

"Speaking of correct answer, other than A does not exist"