Question

why diamond has a high melting and boiling point unlike other covalent compounds? And why is it hard?
Is they a difference between strong and weak covalent bonds if any?

Answer 1

Its covalent bonds are very strong, and there are many of them, a lot of energy would be needed to separate atoms. This makes diamond's melting point and boiling point very high.
The reason for its hardness is also same,
Diamond has a giant molecular structure. Each carbon atom is covalently bonded to four other carbon atoms. A lot of energy is needed to separate the atoms indiamond. This is because covalent bonds are strong, and diamondcontains very many covalent bonds. that’s why Diamond is very hard and storng substance.

Usually the covalent bond is considerd as one of the strongest intermolecular bonds.
But of course it's strength varies from atom to atom.This is because, the bond strentgth also depends on the valency and so many other factors too.
But a clear distinction between strong and weak covalent bonds is hard to define.