If you describe a simple sentence, in comparison with compound or complex one, as (Subject + verb + object), you come too short and you may often be misleading. It is the meaning which should be taken into consideration. Compare these two simple sentences for instance:
*/ My cat drinks milkBoth of them are grammatically correct yet the second sentence is more from Andromeda Galaxy than from our nice Milky Way. Yet here is a funny exercise if you want to make examples like both.
*/ Milk drinks my cat