Things can be counted. We call them Countable Nouns. Examples: students, books, pencils, dogs, flowers, kites, bananas, cars, apples, teachers, and etc.
Things cannot be counted. We call them Uncountable Nouns. Examples: sugar, dust, water, jam, flour, syrup, salt, coffee, ink, sand, air, money, tea, juice, oil, butter, and etc.
Countable nouns we use many.
Uncountable nouns we use much.
Do these online:
Many more exercises: http://www.agendaweb.org/exercises/grammar/countable-uncountable/many-much