Covalently bonded substances fall into two main types:
However, although the covalent bonds holding the atoms together in a simple molecule are strong, the intermolecular forces between simple molecules are weak.
Hydrogen, ammonia, methane and pure water are also simple molecules. All have strong covalent bonds between their atoms, but much weaker intermolecular forces between molecules.
When one of these substances melts or boils, it is these weak intermolecular forces that break, not the strong covalent bonds. At room temperature, simple molecular substances are gases, or liquids or solids with low melting and boiling points.
The slideshow shows how the weak intermolecular forces between water molecules are broken during boiling or melting: