Carbon atoms must bond with 4 other atoms in this structure one of the carbons only has 3 bonds with the other atoms.
Oxygen has to bond to two other atoms so the C-O bond should be a double bond to fulfil both of their bonding rules.
Hydrogen can only bond to one other atom, therefore it cannot make a double bond so that bond must be a single bond. (Carbon there is also bonding to five atoms so changing it to a single bond will fulfil both of their bonding rules).
