How do i draw isomers for alkanes and tell the longest chain for alkanes . i was absent during the lesson?

Given an alkane formula of ${C}_{n} {H}_{2 n + 2}$ you must simply use your skills of geometry.
Once given a formula of ${C}_{4} {H}_{10}$ and higher, isomerism is possible. Butane has 2 isomers, pentane has 3 isomers. The bigger the alkane, the more isomers can be supported.