1) Well, models are vital to use by scientists, because that way they can represent visually and help other people understand the point they are trying to stress about their project. A simple example would be to use a Bohr model to represent a specific chemical, and along with that show how many protons or neutrons does it have.
2)In science a universal law is a statement by which a particular action or a phenomenon would occur ALWAYS in the same way, and will ALWAYS abide to what the law states, while a theory is a guess on what kind of pattern does a particular phenomenon or experiment abide by.
3)The SI units are units of the metric system, and are used everywhere in the world for specific measurements, the kilograms is used when measuring mass, the meters is used when measuring length, the liter is used when measuring a volume of a liquid, and finally the cubic meters or m3 are used when measuring the volume of a solid. and those are the standards as well.
