I was wondering about that too. I think an "action" verb needs to be used. For example, "Ich esse einen Apfel." Eating is an action verb that I am doing on the object (apple). In the practice example, the verb "sein" (to be) isn't an action verb....I think? This is why "Das ist ein Fisch" is correct.
I could be wrong, but this is the only way I can make sense of it at this point. (It would be appreciated if anyone can provide a more technical explanation!)
epicknight7 you're right. It is "ein" because the sentence is in the Nominative case. We are merely saying what "Das" ("that" in English) is.... a fish. If we were performing an action upon the fish this would change it to the Accusative case which then changes the ending of masculine articles (der -> den, ein -> einen). So:
Das ist ein Fisch. = Accusative, no change to ein
Ich esse einen Fisch. = Dativ, masculine article changes by adding -en
So, if the subject performs an action on another object, then you change the ending for masculine articles. We didn't in the example here, so the infitive "ein" is used.
P.S., for those interested, the part of the sentence "ist ein Fisch" is just a predicate phrase, meaning we are adding description to the subject ("Das", in this example) which doesn't change anything.
P.P.S. Yes, "That is a fish" is also correct, both "a" and "one" are definite articles in English so we can interchange them. How to tell the difference in German? Context. If you were yelling at someone for not bringing home enough dinner for two people they would get that you were emphasizing"one" not "a".
Not 100%ly correct:
Das ist ein Fisch. => Nominativ! The verb "sein" does not require an object. that phenomenen is called "copula".
Ich esse einen Fisch. => Akkusativ!
The verb "essen" requires the accusativ for the direct object.
"der Fisch" is a masculine noun. So it changes the article.
"That is one fish" is just as correct as "That is a fish" in English. The more appropriate phrase depends on the meaning you want to convey. You can simply remark that a thing is "a fish" or you could hold it up and say "This is one fish" to indicate that it is indeed only one fish.
You wouldn't say the latter all to often, but it is not incorrect.
http://www.merriam-webster.com/dictionary/copula Yes, strangely when they use the term copula for the verb "to be" they sometimes call the predicate nominative just the predicate which feels odd to me as I learned the predicate to mean the verb with all its complements.
The reason I used the term linking verb is because there are other verbs that can be used this way.
Here they actually list many of the linking verbs which include "become, appear, seem, feel, grow, " Many of those are more likely to have a predicate adjective, which still refers to the subject, instead of a predicate nominative, but "to become" often takes a predicate nominative just like "to be".
If you use that and this in the same sentence, they will use der/die/das for that and dieser/diese/dieses for this. Seperately either set of words can mean this or that. If you have something farther away, you can use : "das....dort" or "das...da" kind of like we might say "that...over there"
There is no difference between this/that in German.
It can seem strange to English-speakers (almost like something is missing), but each language views/describes the world a little differently. Japanese, for example, has one form of "this" and two forms of "that" based on distance. But in German, all three would be translated as "das".
Well, there is a difference in German as well. But if you use the pronoun "das", it can mean both. Thats why you usually say "das hier" (=this) or "das da" / "das dort" (=that) to show the difference.
Instead of "das" you can use "dieser/diese/dieses" (=this) and "jener/jene/jenes" (=that). You need to use the right gender of course, but it sounds more sophisticated ;-)
The verb "haben" (to have) requires the accusative, but the verb "sein" (to be) requires the nominative! Something like this we call Copula.
That's why you have to say: Das ist ein Fisch.
In english the copula does not exist. This is always causing mistakes in translating from German to English and vice versa. For instance you say in English "That's me.", for instance when showing a picture of yourself.
But in German we say: "Das bin ich." and not "Das ist mich."