Ponieważ już na podstawowym poziomie pojawia się to zagadnienie, lecz może być to mylące dla wielu jak i dla mnie było do teraz i przyznam że po wielu latach z angielskim pierwszy raz się o tym dowiaduję )).

To znalazłam na internecie i myślę że powinno to być wyszczególnione w ćwiczeniach o uncountable nouns, dziękuję :

Fruit or Fruits?

Which form is correct?

When it means "a type of food like apples and oranges", the noun "fruit" is usually uncountable, so we use "some fruit", "a piece of fruit" or just "fruit" to express that idea:

You should eat more fresh fruit.
Children usually like fruit but they don't like vegetables.
The shop mainly sells fruit and vegetables.
Naturally occurring sugars in fruit are OK.
I like all types of fruit.
Apples, pears and plums are fruit.
Would you like some fruit for dessert?
I eat a piece of fresh fruit every single day.
There was a bowl of fruit on the table.
Fruit is quite expensive this year.

Sometimes the form "fruits" is used instead of "different types of fruit":
Many fruits and vegetables provide fiber, beta carotene and vitamin C.

You should be very careful with the countable noun "fruit" because it has two possible plural forms: the regular "fruits" and the irregualr "fruit". Thus, it is possible to say "many fruit" intead of "many fruits". Example: Many fruit and vegetables need to be stored in lower temperatures.

When the meaning is "the part of a plant that contains its seeds", the countable form is usually used: There were many bushes with small blue fruits.

When the meaning is "the good results of something", the countable form is also preferred:
We're just now starting to see the fruits of our efforts.

Also remember the following phrases:

the fruits of your labour = the results of your hard work
I'm going to retire soon and enjoy the fruits of my labour.

the fruits of nature / the fruits of the earth = the natural things that the earth produces
There was gratitude for the fruits of nature and hard work.

a tree or a plant is in fruit = it has fruit growing on it
The apple trees are in fruit from May to October.