Since you seem to be taking o lvl this year, I shall just keep
it at that for my answer.
1) Why are the carbon single bonded? Well carbon is an amazing
element since it is in group 4. This means it has 4 valence
electrons. It can opt to form single, double or even triple bonds
with other elements. It does not form quadruple bonds with other
elements though.
As to why the carbon atoms are singly bonded in alkanes, well
alkanes are special because they are known as unsaturated compounds
and in unsaturated compounds only single bonds exist in contrast to
that of alkenes where double C=C bonds exist.
2) By 4 single covalent bonds, it means that the bonds it forms
with other C or H are covalent and not ionic or metallic. The
implication is that the whole alkane molecule is made up of only
covalent bonds and there's no ionic bonds and hence it has covalent
properties instead of ionic properties.
As to why 4 single covalent bonds for carbon in alkanes, I will not
go into that since it is A level and above.
