For quantity:
* Less (for uncountable nouns) - "I have less time today than yesterday."
* Fewer (for countable nouns) - "There are fewer people in the room now."
For size:
* Smaller - "The cat is smaller than the dog."
Important Note:
* "Less" is used with uncountable nouns (things you can't count individually, like time, water, sugar).
* "Fewer" is used with countable nouns (things you can count, like people, books, apples).
* "Smaller" is used for size comparisons.