Accusative after "sein"

Question

I’m lost with the verb sein. I read on multiple websites and according to what I have learned, sein is always followed by nominative.

Examples:

Er ist der Mann.
Er ist ein großer Mann.

not

* Er ist den Mann.
* Er ist einen großen Mann.

But does one say the following?

Es ist nächsten Sommer.
Es ist letzten Sommer gewesen.

Shouldn’t it be:

Es ist nächster Sommer.

My question is, why is it nächsten and not nächster? And is sein always followed by nominative? I’m wondering because it seems like dative is used instead of nominative in the following sentence:

Das ist mir egal.

Answer 1

The premise that sein is always followed by nominative is wrong. You don't even need a substantive, e.g. "Ich bin müde.".

You could say that sein is always accompanied by nominative; but seriously, there is (almost) always a nominative because the subject, which is obligatory, is in nominative. And in case someone raises objection to this statement: yes, there are some exceptions (e.g. "Mir ist kalt."), but this is irrelevant to your question.

So, sein is a verb that has quite a few definitions. This verb can even be used as copula, auxiliary and full verb.

One definition basically expresses an equality between two things. A sentence like "Ich bin Lehrer." means, kinda, 'I equal teacher'. In that case both parts have to be in the same case. Since I is in nominative (as it is the subject of the sentence), Lehrer is in nominative, too.
And this applies to your first example.

Another definition for sein is that something occurs at a certain place or at a certain time. Such a sentence doesn't have a second substantive whatsoever, so obviously there can't be another one in nominative. Your other example (i.e. "Es ist nächsten Sommer.") fits into this group.

The past form of that sentence (i.e. "Es ist letzten Sommer gewesen") is comprised of two sein. The former one is the auxiliary verb for the present perfect, the latter one is the past participle of sein, expressing yet again that something occurs at a certain time. There's no second substantive, hence there's not another nominative.

The full story about sein is sheer endless. But if I haven't missed anything, the only case with two nominatives is the equality that I talked about above. So, only when you can apply 'A equals B', the substantive following sein is in nominative.

Answer 2

1) Adverbials that describe a period of time contain nouns use the accusative (Adverbialkasus). This is why "nächsten Sommer" always stays in the accusative, no matter what happens to the subject, direct object and indirect object:

Ich (Nom.) gebe nächsten Sommer meinem Freund (Dat.) meinen Stadtplan. (Akk.)

Mein Stadtplan (Nom.) wird nächsten Sommer von mir meinem Freund (Dat) gegeben.

The same applies to sentences of the form "X (Nom.) ist Y (Nom.)":

Er (Nom.) ist ein großer Mann. (Nom.)

Er (Nom.) ist nächsten Sommer ein großer Mann. (Nom).

And the same applies to "Es ist gewesen" in the sense of "Es ist passiert":

Es ist letzten Sommer passiert.

Es ist letzen Sommer gewesen.

(This construction doesn't really work with the Präsens, "Es ist nächsten Sommer" makes no sense, though you can say "Es passiert nächsten Sommer" in the sense of "Es wird nächsten Sommer passieren". You can also say "Es wird nächsten Sommer/im nächsten Sommer sein").

2) It's also possible to have sentences of the form "X ist Y" where Y is not a noun, but (formally) an adverb (though there's a number of different names for it): "Dieses Auto ist grün", "Es ist egal". As Y is not a noun, of course it doesn't have a case.

So there is no rule "sein is always followed by the nominative". The correct rule is: "if two objects X and Y are described as equal using sein, then both are in the nominative".

And for some expressions, you can add an indirect object to sein: "Mir ist kalt", "das ist mir egal". Sometimes that even works for two nouns: "Das ist mir eine Lehre".

Answer 3

In Es ist letzten Sommer gewesen, the subject isn't letzten Sommer but Es.

Es is a fill-in subject used when you need something in nominative – as with sein.