Why do you think that it's fairly easy for the sun to fuse hydrogen atoms, but a lot harder to fuse helium atoms?

Answer: Because it involves a process called the proton-proton chain.

Explanation:

Two protons (hydrogen-1) fuse together, producing deuterium (hydrogen-2) and other particles plus energy, Deuterium (hydrogen-2) and a proton (hydrogen-1) fuse, producing helium-3 and energy, Two helium-3 nuclei fuse together, producing helium-4, two protons (hydrogen-1), and energy, Helium-3 fuses with helium-4, producing beryllium-7, which decays and then fuses with another proton (hydrogen-1) to yield two helium-4 nuclei plus energy