Would the sentence "我知道它,但是我忘了“ sound unnatural? As in, 'I know it, but I've forgotten it' (I learnt it sometime ago, but have now forgotten it)

  • see grammars (personal pronouns, 人称代词)on avoidance of third person pronoun 它,since it would represent something mentioned before, there is no need to use a pronoun, 我知道 is usually sufficient, in case of doubt as in below answer use demonstrative pronoun (指示代词:这、那)+( CL 个 or another noun,e。g。事情 w or w/o CL (件)) – user6065 Jun 18 '17 at 20:59
  • CCG Ch.25 abbreviations and omissions. "Pronouns in particular are regularly omitted and the third person neuter 它 "it" occurs quite rarely, since it is by definition a reference back to something already identified. Questions with their answers give clear examples of this feature." – user6065 Jun 19 '17 at 6:13
up vote 1 down vote accepted

We seldom say 我知道它

我知道這個/那個 may be better.

However, the best answer depends on the context.

  • Ah, so '我知道这个,但是我忘了' would sound natural? Thanks – Somniare Jun 18 '17 at 20:18
  • You are welcome! Yes, that sounds more natural to me. – velut luna Jun 18 '17 at 20:19
  • 2
    '这个我知道,但是我忘了' is the most natural way in day-to-day conversation. – Tang Ho Jun 18 '17 at 21:42
  • Sorry for chimming in but is 忘了 more natural than 忘记了? – Blaszard Jun 19 '17 at 17:28

Both "我知道它,但是我忘了" and "我知道这个,但是我忘了" sounds unnatural to me. I have no idea what you are talking about until I saw the English explanation: I learnt it sometime ago, but have now forgotten it. To me, the most natural way to say this is to say it with details, like "我知道这个,但是我忘记具体的内容" (I knew this, but now I forgot all the details) or even "我记得我看到过这件事,但是其他的都忘记了" (I know I have seen this, but I cannot remember anything else about it).

It depends on the context.

For example, if you are talking about some object you've seen before, maybe it seems weired, because in this case 忘了 is too vague for any specific meaning. You need to mention what you forgot -- maybe the place, the shape, etc.

On the other hand, if your friend is mentioning something old, and ask for your confirmation of knowning it. You have already forgot it, but suddenly think of it after this hint. Then, 我知道它,但是我忘了 seems more natural.

In Mandarin, 知道 is more complicated than "know". 道 itself mean "nature's rules". So implicitly, if you know the nature, you cannot "forget" it. It is almost similar to "enlighten".

That's why it is not proper to use 知道 for something you already know/learn/heard. You must specify the subject:

  • 我听过这个说法, 但是我忘了具体的内容
  • 我曾经读过这本书,但我现在忘记了它说什么。
  • 我学过这个算法, 但(现在)我忘了。

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