Answer to Question 1
The four fundamental forces in the Universe are gravitation, electromagnetism, the strong nuclear force, and the weak nuclear force.
At the instant of the big bang, all four forces were joined into a single unified force. During the very first stages of the big bang, they began to split. Gravity split away from the other three forces first. The strong nuclear force then separated from the electroweak force. Finally the electroweak force split into the electromagnetic and weak nuclear forces.
When the strong and the electroweak forces split, a huge amount of energy was released. This energy inflated the Universe by an immense amount, and helps explain several aspects of the inflationary hypothesis.
Answer to Question 2
The fusion of hydrogen into helium in the Sun proceeds in three steps known as the protonproton chain. The first step in the chain combines two hydrogen nuclei to produce a heavy hydrogen nucleus called deuterium. The second step forms light helium, and the third step combines the light helium nuclei to form normal helium. During the process, positrons, neutrinos, and gamma rays are formed and energy is released as the particles fly away.
Fusion can occur only in the core of the Sun where temperatures are pressures are high enough. Because particles of like charge repel one other, high temperatures are needed to give particles high enough velocities to overcome this Coulomb barrier and fuse together. High densities are needed to provide large numbers of reactions.
Only a tiny fraction of the hydrogen atoms fuse into helium, and the nuclear reactions in the Sun are spread through a large volume in its core. Any single gram of matter produces only a little energy. A person of normal mass eating a normal diet produces about 3,000 times more heat per gram than the matter in the core of the Sun. Gram for gram, you are a much more efficient heat producer than the Sun. The Sun produces a lot of energy because it contains many grams of matter in its core.