The name of a compound tells you which elements it contains. The ending of the name is a clue. A name ending in -ide usually means the compound contains just two elements. The name of the non-metal changes: chlorine becomes chloride, oxygen becomes oxide, bromine becomes bromide and sulfur becomes sulfide. So sodium chloride contains only sodium and chlorine, and magnesium oxide contains only magnesium and oxygen.
A name ending in -ate means the compound contains oxygen combined with at least one other non-metal element, as well as the metal. Sulfate contains sulfur and oxygen, nitrate contains nitrogen and oxygen, and carbonate contains carbon and oxygen. So sodium sulfate contains sodium, sulfur and oxygen, and calcium carbonate contains calcium, carbon and oxygen.
There is one common exception. Hydroxides, such as sodium hydroxide, contain oxygen and hydrogen but still end in -ide.