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I am volunteering for an ESL program and one of my students wrote:

I know is very rare.

As a native English speaker, I know that "it" is required. But I'm struggling to come up with an explanation of why.

Is the subject/verb/object "I know it" or "it is rare?" The two nouns & two verbs is confusing me.

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General Reference. Subject: I, Verb: know, Object: [the thing known] - in this case, [that] it is very rare. –  FumbleFingers Oct 30 '12 at 3:57
    
Welcome to our ESL Q&A: area51.stackexchange.com/proposals/41665/… –  Kris Oct 30 '12 at 4:04
    
Have you thought about including transitive verbs in your explanation? –  J.R. Oct 30 '12 at 4:15
    
Thanks, that helps a lot. Sorry if it's a dumb question, but I've been trying to figure this out for several days and couldn't seem to find the answer with Google. –  Benjamin Herreid Oct 30 '12 at 4:21
    
Ask yourself what it is that is very rare. –  Barrie England Oct 30 '12 at 7:53
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closed as general reference by FumbleFingers, tchrist, RegDwigнt Oct 30 '12 at 10:15

This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information.If this question can be reworded to fit the rules in the help center, please edit the question.

3 Answers

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What you have is basically a sentence that occupies the position of a noun phrase:

I know [this fact].

I know [it is rare].

So, the reason that "it" is required is basically because in English (as opposed to some other languages), sentences must generally have an overt subject. Just because this sentence is actually a sentence-in-a-sentence (traditionally, the label used for a sentence-in-a-sentence is "clause") doesn't essentially make any difference: structurally, it's still a plain old sentence and needs an overt subject.

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Thanks, "noun phrase" is the concept I was trying to figure out but didn't know the name of it to look it up. –  Benjamin Herreid Oct 30 '12 at 12:45
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The question of a need for the pronoun arises because of a presumptive analysis of the sentence.

*I know is very rare.

Cannot be necessarily interpreted as "I know it is very rare." without additional contextual information. It could have been "I know (anything) is very rare." What is the theme of the sentence? Something foregone referred to by the pronoun? An existential it? Yet something else?

I'd parse it as [I know][is (whatever)]. That shows the sentence lacking its thematic subject. It requires a noun/pronoun/existential it to complete.

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I had thought of that, too... e.g., I know the steak is very rare – we should put it back on the grill. –  J.R. Oct 30 '12 at 4:55
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Without the "it", then the subject of the verb "is" can only be "I know". That is, the sentence is saying that "I know" is very rare. What does that mean? That knowledge is rare? That people saying "I know" is very rare? It doesn't make much sense, and in any case almost certainly doesn't mean what you intend.

But in "I know it is very rare", the subject of "is" is now "it". So the sentence logically means: "I know" something. What do I know? I know that "it is very rare".

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