Question

why can carbon forms only covalent

Answer 1

carbon is a semiconductor. It has four electrons in its last orbit. So it can make a coventant bond

Answer 2

Hi kabir
To gain stability ,it should lose its valence electrons ,gain four more electrons or share its four electrons with other atoms.
carbon atom has no tendency to lose its four electrons or gain its four electrons .so that they share their electrons with other atoms and there fires forms covalent bond
Hope this help you friend