Hydrocarbons, functional groups and isomers
A prefix gives the carbon count, the functional group gives the family, and isomers share a formula but differ in structure.
A worked example
Propan-1-ol is C3H7OH: prop- for three carbons, -ol for the hydroxyl group. Pentane, C5H12, has three structural isomers.
The mistake people actually make
Alkanes are CnH2n+2, alkenes CnH2n and alkynes CnH2n-2. Saturated chains undergo substitution; only double and triple bonds undergo addition.