I have two examples:

  1. ‘I haven't been able to find my keys for two years. (which means I don't have them now)

  2. ‘I hadn't been able to find my keys for two years. (which means I finally found them)

But how should I write them if I want to replace be able to with could?

  1. ‘I couldn't have found my keys’
  2. ‘I couldn't had found my keys’

Is it correct? I think the second one is incorrect. So is there any correct form of it?


This is my understanding of what your example sentences mean:

  1. I haven't been able to find my keys for two years. (which means I don't have them now, or that I have just found them) (Present Perfect)
  2. I hadn't been able to find my keys for two years. (which means that there was a period of time in the past, prior to some other past event or occurence, during which I was not able to find my keys. It is possible that I still haven't found the keys) (Past Perfect)

Now, could we replace be able with could?

  1. Sentence 1 is built using the Present Perfect tense, which is formed thus: HAVE + [verb in the past participle form]: "haven't + been".

This scheme is impossible with could because it is a special form of verb: an auxiliary verb. It is a past-tense form of the verb can.

Auxiliary verbs are "defective". The verb can has no participle forms: you can't say "I had canned play tennis but then I broke my leg" or "I am canning to dance!". So we can't take have (or had) and add the past participle of can to it, since there's no such thing.

When you add have after could, it has an effect quite unlike that of the Present Perfect. The construction serves to express disbelief about a past event or action:

You couldn't have done it! (I express disbelief in what you did. )
I couldn't have found my keys! (I express disbelief in that I have found my keys)

Your example 4 is easier to explain: we do not use had after could, so

I couldn't had found my keys.

..is ungrammatical.

What could we do with could? Only one thing:

I could not find my keys for two years.

What would be the meaning? In the past, there was a 2-year period during which I wasn't able to find my keys. Do I have the keys now? Who knows! How long ago did this 2-year period end? Who knows! So it's better to use be able to to express more "advanced" meanings.

  • Thank you for explanation and getting my point. This question came to my mind to know how to show a period of 'being unable' from a point in the past till now (present perfect) and from a point in past until some other point in the past (past perfect) So you mean that we should use ‘I couldn't find my keys for two years’ in both cases. Now how can we distinguish the difference? So you suggest not to use 'could'. Am I right?
    – vahid3561
    Jan 7 '15 at 15:34
  • @vahid3561 - the point is that with could you would glean no difference from the sentence itself: you would have to guess from the context (from the sentences that go before and after the sentence with could). That's why there is such an expression as be able to: it can participate in the Present Perfect and Past Perfect constructions, allowing to pack more information in the sentence. Jan 7 '15 at 15:37
  • You're welcome, @vahid3561! Jan 7 '15 at 15:41

I am not native but as far as I know

could + have +past participle form has meaning of past form of possibility

It could have been worse ( but it did not become worse)

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