Past 了 used with a simple object I've read in many grammars and have seen in many places that, when we have a verb-了, its object has to be a complex one (taking a number, an attribute, etc.), for example:


  我吃了一个苹果。
  我吃了那个红苹果。


These grammars also state that when the object is a simple one, the only possible and correct 了 would be a sentence 了 or a double 了, as in:


  我吃苹果了。 (ambiguous)
  我吃了苹果了。


But I have heard and seen in many places people saying (and accepting) sencences like:


  我吃了苹果。


Once, a native Chinese friend of mine said that when he heard these two sentences:


  我吃了苹果。
  我吃苹果了。


He felt that in the former he knew what apple was being talked about. After that I started to wonder if it would be possible to accept sentences with a simple object and if the verb-了 in those cases would define the object in the same sense as the 把 structure does.

Any comment would be appreciated.
我吃了苹果。
我吃苹果了。

In practice, both sentences are applicable depending on the context. See these examples:

A: 你吃了什么东西？
B: 我吃了苹果。
A: 你吃什么了？
B: 我吃苹果了。