How do you indicate that a methyl group is attached to the second carbon atom in a hydrocarbon?

To indicate that a methyl group is attached to the second carbon atom in a hydrocarbon, you use the IUPAC naming system. You would name the base hydrocarbon chain and then use a locant (number) to specify the position of the methyl group. For example, if the base hydrocarbon is butane and a methyl group is attached to the second carbon, it would be named 2-methylbutane. The number "2" indicates the position of the carbon to which the methyl group is attached.