Is the S+V+了+O+了 structure only works for certain verbs?

Question

Hi everyone… I’m back with another 了1 question. I am sorry if my questions sound dumb but I’m really confused.

This article said that 了1 can be placed at the end of sentence if the object of the sentence is short, for example:

我吃饭了。

我上课了。

老师问问题了。

丽丽到火车站了。

However, I found another article that said:

If the countable noun is not quantified, 了 must be used twice. For example:

我吃了饭了。

我上了课了。

我唱了歌了。

I’ve found the answer (I don’t know if we’re allowed to attach link from another forum here), someone said that “我吃了饭了” is just an emphatic version of “我吃饭了”.

I then tried to look for more examples by changing “吃饭” into “问问题” (because both verbs have short object) but couldn’t find anyone who says anything like ”老师问了问题了”.

People seem to only use the “S+V+O+了 (老师问问题了)” structure in the case of “问问题”.

Does this mean the S+V+了+O+了 only works with certain words (the ones that listed as “V//O” in the dictionary)?

I’m not sure, though, since “我买了书了” seems to be just fine (I can’t confirm this, though. It’s just my 语感, I could be wrong).

Answer 1

If the countable noun is not quantified, 了 must be used twice.

Not true. The 了 immediately following the verb should be replaced by "過": which indicates "the person in subject ever performed the action described by the verb"

我写過字了 - I have written the words (already).

我上過课了 - I have attended class (already).

我唱過歌了 - I have sung (already).

In the above examples, "過" indicates "the person in the subject ever performed the action described by the verb". And the "了" at the end of the sentence indicates the completion of that act.

Hope this helps.

Answer 2

In general, 我吃了饭了 is a complete sentence, while 我吃饭了 may mean something else。

For example,

妈妈在电话里对爸爸说：“你快点儿回来吧。我们吃饭了，不等你了。”

Another example without the second "了" is,

我们吃了半截饭，老板就叫我们出去干活。

means we have not finished lunch yet.

吃了，吃着，吃过，吃完， etc. may have different meanings in different contexts.