In English, a monotheist may say there is only one god, namely Hashem.
The Hebrew would be something like, "There is only one elohim, Hashem".
There is a problem with that statement. Many bible verses claim that Hashem is not the only elohim around. Angels, judges, rulers, are called elohim too.
Maybe Hebrew simply does not have a word that exclusively mean god. Their next best word, eloah/elohim, can mean too many things. So, Hashem is the only _ ? What should I fill in the blank? He's not the only elohim. Too many bible verses claim there are other elohim too. Angels are elohim too. Judges are elohim too.
In other languages we have classes of being, called gods. The monotheists can simply claim that Hashem is the only member of those beings. In Hebrew, Hashem is the only member of what?