“sweet” is a countable noun as you can say “I have one sweet” or “I have ten sweets”.
“coin” is another countable noun as you can say “there is a coin on the table” or “there are many coins on the table”
“homework” is an example of an uncountable noun.
In English you cannot say “the teacher gave me many homeworks” instead you can say “the teacher gave me a lot of homework”
I have attached a useful list of uncountable nouns for you memorise
List of Words Without Plural Form (Uncountable Nouns)
Is ”Person” an uncountable noun
There is / There are / There isn’t / There aren’t
I hope this helps!