What are the primary roles of each of the fundamental forces?
The four forces are the strong force, the weak force, electromagnetism and gravity. Each plays a different role on different scales.
At the highest energy (the strongest) is the aptly named 'strong' force. It binds the components of protons and neutrons together and also has a role in binding the nucleus as a whole. It's range is effectively less than the size of the nucleus and it diminishes very quickly with distance. It is also strange because it is repulsive at very short scales, but attractive at 'larger' distances (still around the order of
The weak force is primarily involved in altering the character of charged particles (as most of its bosons are charged, which is unusual). This means it is involved in beta decays where, for example, a neutron 'splits' to form a proton and an electron.
Electromagnetic forces only affect charged particles, and are responsible for most of the large-scale physical effects we experience - light, sound, heat, chemical changes etc. This is because, though much less powerful, it is the first one that has infinite range (though it diminishes significantly with distance.)
Finally, gravity. This is a problem for physics. It has infinite range (much like electromagnetism) but is stunningly weak, just impossibly feeble (a tiny fridge magnet can pull harder on a photograph than the whole planet pulling in the opposite direction) yet no one knows why. It is also the only significant force on cosmological scales i.e. it dominates galactic motion.
So far the first three have essentially been incorporated successfully into an overarching theory (QCD) but gravity has never been included. I don't really understand why, but a very clever person once told me that the first three forces are fundamentally numerical theories (statistics based on quantum theory) whereas gravity is geometrical (or topological.)
Does this help?