I need a method to find all subgroups from any finite group. For example, the subgroups of $D_4 $ (of order $4$): $\{(1), (1 \ 2 \ 3 \ 4), (1 \ 3)(2 \ 4), (1 \ 4 \ 3 \ 2)\}$.
Okay, I can understand this but the others .. how we can get them? Is there any theorem or something?