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While I was reading a grammar book I came upon this sentence:

'...but it has changed because the pronoun in front of it has changed...'.

Why does the author use 'has' rather than 'is'? Note: perhaps this is an elementary question, but I don't know the answer.

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up vote 4 down vote accepted

Both uses of have are auxiliary verbs of the Perfect construction. Changed is a past participle, which is the other part of the Perfect construction.

The reason why is is not used is because the construction is not Passive, which requires a form of be before a past participle, and a causative transitive verb change, with an agent (i.e, it was changed by someone, or some event, or some force) as well. This is not the case here.

These Perfect uses of change are not transitive and have no agent, so they can't be Passive -- there was a change, and the cause was another change. That's all, really. For more details on the Perfect construction, look here.

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When you're talking about the act of changing, you have to say "It has changed" (and you're talking about the time that it changed). But if you say "It is changed", you are talking about the state after the act of changing.

For example:

The policy has changed (referring to the time that it changed). And the policy is changed now.

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I suspect the past tense is used to emphasize that it happened before the book was written as part of language drift; "...it is changed because..." would imply to me that new policy was being set by the author.

This reminds me of the difference between "will" and "shall" in government documents, where "shall" implies a command and "will" a wish. (As in "The committee [will|shall] report in three months.")

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There's no past tense in the sentence. Both tensed verbs (has) are present tense. Past would be had. The construction is called (Present) Perfect. – John Lawler Apr 23 '12 at 22:05
Thank you; you are right. However, I'm right in spirit; present perfect is used to "say that an action happened at an unspecified time before now" (from englishpage.com/verbpage/presentperfect.html, a site I cannot vouch for but which validated my gut feeling). – Jennifer Davis Apr 23 '12 at 22:10
That's "(b) The Existential sense of the Perfect, used to indicate the existence of past events, (e.g: I have read Principia Mathematica five times)" from the link. – John Lawler Apr 24 '12 at 1:17

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