The German sentence may lead you astray. Knowing the English translation "It is a potato" gives you "Es = It" + "ist = is" ... But if you only have the German sentence (written or oral) you have a different option. "Es = it/the child" + "isst = eats"; if Duolingo overlooks "isst" with a double s, it accepts your version, which unfortunately for you is wrong. Sorry; hope this helps
The word is grammatically treated as feminine, but the potato is still an object, not feminine or masculine. It's the same with the gender of most other words - you don't refer to doors, tabls, spoons, etc. as "he" or "she." This mistake (prominently among others) has often been used in TV shows to indicate someone is French and not quite fluent in English.
Actually, you do refer to potato and door with sie and to table and spoon with er in German. That is quite similar to French with the exception that we have (for whatever reason) 3 randomly distributed genders instead of only 2.
To be honest RynD's question got me thinking quite some time until I noticed that es is only used as a placeholder here. You can see it when you put Kartoffel into plural. Then the sentence would look like this:
Es sind Kartoffeln.
So the verb is conjugated with respect to Kartoffel and independently of es. A search reveals it is apparently a "formal subject with sein":
"einen" is the "a" form of "der" in the objective case. If you are performing an action upon an object, the object's article "ein" will become "einen". 'Ich bin ein/der Mann." is "I am a/the man." "Ich esse einen/den Mann." is "I eat a/the Man." Bad example, but I couldn't think of another masculine word haha I feel like Tobias.
This is my first time running into the word "Kartoffel", and mousing over reveals the definition as "potato/conk/hooter". I just... what even? In this sentence the def. is obviously 'potato', but can someone please clarify the other uses of this word? Should I refrain from complementing people on good-looking "potatoes"?