Is it true that the law of conservation of mass and law of constant proportion are the two important laws of chemical combination?

The above statement is true.

Law of conservation of mass: According to this law- ‘Mass cannot be generated or destroyed in a chemical interaction.’

Law of constant proportions: According to this law- ‘In a chemical substance, the elements are always present in precise amounts by mass.’ Nitrogen and hydrogen, for example, are always present in a 14:3 mass ratio in ammonia, regardless of the source from which it is obtained.

Hence, the two important laws of chemical combination are Law of conservation of mass and law of constant proportions.