Carbon has 4 electrons and hydrogen has 1 electron in its outermost electron shell. a carbon atom can form covalent bonds with how many hydrogen atoms

This one might be easier to do if you drew a picture of the atoms. I have drawn them for you and attached the screenshots below (The black dots represent the Valence electrons). Carbon has 4 valence(outer) electrons, but most atoms seek to have 8 valence electrons. Some molecules, like carbon, obtain these extra electrons by sharing them with other atoms. When electrons are shared between two atoms that is called a covalent bond.  So, carbon needs 4 more electrons to make it stable. Each hydrogen has 1 valence electron, but seeks to have two valence electrons to make it more stable. In the final image you can see that carbon can make 4 covalent bonds with 4 hydrogen atoms. I hope everything made sense! If not comment with any questions below!

They can measure  the amount of energy gained or lost