"The fruit is in the basket."
Translation:Het fruit zit in de mand.
No it doesn't work. "Zit erin" only works when you are not specifically referring to something within the sentence. Only if you are referring to something you already know.
"Is the fruit in the basket? Yes, it is in it" - "Zit de fruit in de mand? Ja, het zit erin"
"Are the books in the bag? Yes, they are in it" - "Zitten de boeken in de tas? Ja, ze zitten erin"
In these setences the "er" have the same function as "it" in the English sentence. They refer to the basket and the bag. When the basket or the bag are in the sentence itself there is no need to refer extra to them. Hence:
"The fruit is in the basket" - "Het fruit zit in de mand"
"The books are in the bag" - "De boeken zitten in de tas"
You can also add "er" (optionally) if the subject is not a definite object, often the case with questions. For example:
"Wat voor fruit zit er in de mand?" - What kind of fruit is (there) in the basket? "Hoeveel appels zitten er in de mand?" - How many apples are (there) in the basket?
It's slightly more complicated. "Fruit" is mostly used for the food, while "vrucht" is more of a botanical term (though also sometimes used for food, especially in compound words, like "vruchtensap").
Beside that, it's also important to note that "fruit" is uncountable (doesn't have a plural form), while "vrucht(en)" is countable.
Thank you for the clarification, Simius. Perhaps you can help me with something that's bothering me. Is the sentence "Het vrucht wordt van de boom geplukt" a correct sentence, even though "vrucht" is a "de" word? Or would it be correct if it said "de vrucht ...." instead of "het vrucht"? Or can't "vrucht" be used here at all?