"This is a fish."
Translation:Das ist ein Fisch.
i might be wrong because i'm new to german but from my very little experience on duo i think the rule is that you use "ein" when it's referred to the subject of the sentence, and "einen" when it's referred to the object. in this case both "das" and "Fisch" are subjects
You are correct. In German, the subject is called Nominativ and the object Akkusativ. There are two more cases (Dativ and Genitiv), but we haven't learned those yet. (I am not a native, but have had German at school and know Latin, which has the same cases)
Addition: the cases have different endings, depending on the gender, case and wether it is singular or plural. However, sometimes they look alike
Yeah, it should be "Diese ist ein Fisch."? This is Diese(r/n), that is Das.
Firstly, Das=that/this and Es=it, a little difference in meanings; Secondly, noun after Das is nominative, but after Es is objective. You can see the changes in 'ein'&'einen' if the noun is masculine, e.g, 'Das ist ein Fisch'; 'Es ist einen Fisch'.
Fisch is a masculine word is german, right? So why I need to use 'Das' instead of 'Der'?
'Das' here is a noun, just like 'Ihr' 'Er'or 'Ich', not as 'the' in English. When it acts as 'the' before some nouns, there are 3 gender forms; but when acting as a noun, it never changes. For example, 'Das ist ein Zeitung'(Feminine) should also be ok, I guess.