The combine appear to be human shaped and speak English, but we never see them take the helmets off. I understand that the soldiers in half life 1 were not the combine, but in half life 2 they were. I don’t even fully understand how they got there in the first place, but who are they. Are the Combine humans or are they aliens?
The short answer to this is yes, the combine soldiers you see in the game are human, or at least somewhat human, but the combine is not of human origin and is not governed by humans. The black soldiers in Half Life are known as metrocops and they are either synthetic humans or humans that have been augmented to essentially be slaves to the combine.
For those who have played all of the expansions, you will discover what one of the leaders of the combine looks like, they are like grub worms. They enslave various races and species to make them do their bidding. So to answer the question more correctly, no the combine are not human, but the soldiers you see in the games likely are. I hate to be providing wikipedia pages as sources for detailed documentation, but the wiki page on the combine is actually really good and will tell you everything that you need to know about the combine from the Half Life series. Knowing a bit more about them actually makes the game a lot more enjoyable. To see how cruel they are and what it has done to the remaining human population of the world.
Anyway, check out this page for much more detail on what the combine is and how they work https://en.wikipedia.org/wiki/Combine_(Half-Life)