Can an HOA really enforce their rules? If they say I can not build a shed on my property, can they really stop me?
The answer is "Yes" they can by obtaining a court order to prohibit you. However, without that court order, their "prohibition" depends on your cooperation. In effect, particularly if the HOA is wrong - an occurrence that happens frequently - you can resist them. At that point, one or both of you can seek relief from the courts or you could both do nothing but continue to debate the issue.
So for instance, the Board says you can not install that basketball goal in the front yard. You do it anyway. They send you threatening letters. You ignore them. This goes on for months. One day, you get served with a lawsuit from the HOA. A court can then decide who is correct based on the common understanding and meaning of the "deed restrictions" or "CCRs" -Covenants, Conditions and Restrictions. If you are wrong, you could be responsible for the HOA's attorneys fees. Most people are loathe to embrace that risk.