I am trying to remember whether the sentences below are correct:

  1. I don't know this guy being able to complete this task.
  2. I don't know this guy can complete this task.

I think the first one is correct. (I know they don't mean much but I am asking in grammatical manner.)

I am also trying to remember how to reduce the "who" within the following sentences:

  1. I don't know this guy who has been doing this all along.
  2. I don't know this guy who has done this all along.

Neither of the first two is correct. It should be

I don't know if (or whether) this guy can complete this task.

"this guy can complete this task" needs a subordinating conjunction to be added onto the sentence "I don't know" as a dependent (or subordinate) clause.

  • Lets say if I say This man who can do a lot of things is my friend. In this sentence, how would you reduce the can?
    – Tarik
    Aug 25 '11 at 13:56
  • "I don't know whether this guy can complete this task" has a completely different meaning. What Tarik was trying to find was a shorter way of saying "I don't know the guy who can complete this task."
    – herisson
    Oct 3 '15 at 21:10

Regarding OP's first question, @mmyers's answer is accurate and complete.

Re the second question (which should have been asked separately), trying to remove "who" is probably both awkward and pointless. Better to remove the clumsy duplication of "this".

Almost certainly the context doesn't include the speaker actually indicating "this guy" (as opposed to any other guy who happens to be around, and quite possibly the actual one being spoken of isn't even present). That first "this" is an informal / slangy substitute for "the", which jars when the second "this" really is something in the "here and now" which is being spoken of. Change it to "the".


The answer was given for the first questions..

But for the second you cant reduce it like that. Reduction is not using relative pronoun only using the verb..

There is a man who is sitting on the bank in the garden. There is a man sitting on the bank in the garden.

If you use who in the meaning of whom you can omit it..

The students who(m) the teacher got angry with were innocent. The students the teacher got angry with were innocent.

As you see you can not change the tense in your pair there is no reduction.. If required I can tell the other reductions about this

  • Please keep continue telling me the other ones especially the ones such as can, be able to, should etc. how I am gonna use them if there is a reduction.
    – Tarik
    Aug 25 '11 at 13:54
  • 1
    Actually I asked my question incorrectly in my original question. What I was trying to ask is such a sentence and reduction of the who : There is a man who can win this trophy, I saw a person who is able to do all the works here, I know a person who could handle all these stuff etc.
    – Tarik
    Aug 25 '11 at 14:00

You can't reduce them, because when the relative clause has an external subject, a non-finite form of the verb has to be used (such as the present participle or past participle). But the modal verb "can" is defective and lacks non-finite forms. It's true that "be able to" is commonly used as a substitute for the infinitive of "can." But "being able to" cannot be used as a substitute for the present participle in relative clauses; it sounds awkward to native speakers. However, you can use adjectives that have similar meaning as a substitute.

Your example sentences don't sound very natural, and the meaning isn't totally clear, so I'll list slightly different ones.

  • One day, a person who can complete this task will appear. (unreduced)
  • One day, a person able to complete this task will appear. (uses "able to" + plain form)
  • One day, a person capable of completing this task will appear. (uses "capable of" + -ing form)

Regarding the second set of questions: in general, distinctions of tense are neutralized in reduced relative clauses, and the perfect aspect cannot be used. Your options are to not reduce, or to lose this information (or to shift it to the main verb). Since the tense of the unreduced relative clause is different for that of the main verb, the best choice is to not reduce.

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