"The dolphin eats fish."
Translation:Le dauphin mange des poissons.
Fish is both singular and plural in English.
If the English sentence was referring to a single fish, it would say, "The dolphin eats a fish." By skipping the article ('a'), the plural noun is implied.
"Fish," is the plural of "fish," in English.
"Fishes," is the plural of "fish," in English.
Both of those statements are correct. "Fish," is the more common plural and refers to multiple "fish." "Fishes," is less common because it specifically refers to multiple species of "fish."
"le poisson," is the singular noun for "fish" in French.
"les poissons," is the plural noun for "fish" in French.
I have no idea what on earth you're trying to say with "poisson and poissons basically mean the same", but I just thought I'd clear up those basic facts for you.
As I said in my comment above, we know the English sentence is using "fish," in the plural sense because there is no article ("a").
When an adjective is placed before a plural noun, des becomes de.
Le dauphin mange des poissons. ~ No adjective.
Le dauphin mange de petits poissons. ~ Adjective before noun.
Le dauphin mange des poissons roses. ~ Adjective after noun.
Because le dauphin is the noun for the dolphin regardless of the mammal's gender. There is no feminine noun. La dauphine actually means something else.
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