Question 1: Why does the spectrum of hydrogen consists of
many lines even though hydrogen atom has only a single electron?
Experimentally, it is observed that hydrogen atom has a number of energy states despite the fact it has a single electron. The other energy states remain empty when the hydrogen atom is in the ground state.
When energy is supplied to a hydrogen atom, its electron jumps up in the energy levels, depending on the amount of energy it absorbs. After some time, the electron jumps back to the lowest energy state but this happens in a number of jumps depending on how energy was absorbed by the electron and to which energy state the electron had raised. When it falls back to the ground state it emits energy radiations with each jump and therefore, spectral lines of different wavelengths (colors) are emitted. This is the reason we see a number of lines in the spectrum of hydrogen.