One topic that causes a lot of trouble for second language learners of English is countable vs. uncountable nouns. The problem is that counting things is often taught as a grammar problem. It is actually a problem with knowing exactly what words mean.
What do the words mean?
Bread is uncountable, because it is a material, not an individual thing. Children in primary one are shown a picture of a loaf of bread and told incorrectly that it is "a bread". Then later they are told not to use "a", which means one, in front of this uncountable word. The poor child then starts to memorise a list of uncountable vs. countable words and many never learn the real language point, which is... If it has been counted it is countable.
Bread is baked in large pieces called loaves, one of them is a loaf. Then they are cut into smaller pieces called slices. If a small piece falls off a slice it is called a crumb.
One loaf of bread
Two slices of bread
Three crumbs of bread*
(*Usually we say three bread crumbs, but both are correct.)
Instead of memorising countable noun lists, a student needs to learn what the word means. Is it the thing (a loaf), what the thing is made of (bread) , or a measurement of the thing (1 kg)?
Why learning lists does not work
I have heard teachers tell their students that hair is uncountable, so anything made of hair is uncountable. Hair was on the list of uncountable things. This list is wrong because the teacher is trying to create a difficult-to-learn grammar rule to cover confusion with word meaning.
I have a beard. (correct)
I have beard. (incorrect, but students are taught to write this.)
Hair is countable if you can count it. Beards are easily counted, 1. Girl's pigtails and ponytails are also easily counted, usually 1 or 2. Someone who is bald and has only three surviving hairs also has countable hair, 3. Since there are so many individual hairs on a person's head they are treated as an uncountable material, like bread or water.
She has hair on her head.
There is water in the cup.
I ate bread for lunch.
Some of my students tried to count all the hairs on a girl's head. They undid her braids and tied her hair into many small pony tails. Then a different girl counted the hairs in each ponytail. After about forty minutes they added up the result. Her hair had become a countable number of individual hairs.
She has 88,158 hairs on her head.
Grapes are treated the same as hair; they are countable because you can count them. The phrase two bunches of grapes counts bunches when we have not bothered to count the actual grapes. Normally we do not use a plural form, like grapes, for uncountable words. Since we can count the grapes easily we add an "s" onto the end. We can pull three grapes off the bunch and then those three grapes are countable.
How do you count or refer to uncountable things ?
To count the uncountable, you count the thing they are used to make or count a unit used to describe how much there is. Water is a material; cups/ bottles/ drops of water are counted. Money is a general concept and can't be counted; dollars are counted.
How much water did you drink? 2 bottles, 1 mouthful...
How much money do I need to pay? 10 dollars...
How much ice-cream did you buy? 1 cone, 1 cup, 1 box, 5 litres...
Saying, a water or a money makes no sense. They are a material and a concept. You can have a hair or a grape, but you might treat them as uncountable if you have not counted them.
Since the word grapes is plural, grape is considered countable. It is countable until they are crushed and made into grape juice. Uncountable things, like happy, water and grape juice have no plural form. They don't have a number before them or an "s" at the end of the word because they cannot be counted. A grape with a skin around it and fruit inside is an individual countable thing. Crushed grape juice cannot be counted, glasses of it are counted.