Question: are all subgroups of abelian groups abelian?
Answer: Yes, subgroups of abelian groups are indeed abelian, and your thought process has the right idea. Showing this is pretty easy. Take an abelian group G with subgroup H. Then we know that, for all a,b∈H, ab=ba since it must also hold in G (as a,b∈G≥H and G is given to be abelian).