I'm a little confused. I know that "fruit" is a singular noun that can also work as plural, and that we always have to conjugate verbs in singular (Am I correct on this one?). So, how do we know if they refer to one fruit or several? And how do we know if they are talking about one type of fruit or more than one? I don't think people always name the fruits to differentiate these two cases. Thanks.
It context based. As it is in english. " rats,the fruit is rotting." Usually means more than one type of fruit. Otherwise you would often specify if it is one kind of fruit. "Rats the apple is, or apples are, rotting". But yeah if they dont specify you just have to guess from context.
So, if want to say "the mouse is in the table" it is right to say "de muis is in de tafel" but "the mouse is under the table" should be "de muis zit onder de tafel". Is it? I'm Brazilian. In portuguese, "the mouse is in the table" do not tell us where in the table it is. So I do not see that like Positional. But I have the impression that it doesn't work like that in English and Dutch...
You're right, the Portuguese 'em' can be "in" ('dentro') and "op" ('em cima'), in Dutch you have to be more specific than in Portugese.
We would only say "in de tafel" if the table has a drawer, otherwise it would be "op" (em cima) or "onder" (debaixo).
With a mouse we would usually say "de muis zit in/onder/op de tafel". If we could see that the mouse was standing up or laying down we might say "de muis staat/ligt in/onder/op de tafel". This is because a mouse is an animal, it can move, it can change its own position. My comment about using "zitten" for 'being positioned inside something else' refers to objects that are put there.
In Dutch "het fruit" is always singular grammatically, but it refers to multiple items of fruit. If you want to talk about one fruit you have to say "het stuk fruit" (the piece of fruit) or "de soort fruit" (the sort of fruit). The verb has to match the noun grammatically, so "het fruit zit".