has changed and is changed

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.

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.

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.