Guid

1- When use PermaLink "true" and when use it "false"? I use iWeb to generate the rss and they put "false", why?

"True" is used when it's a reliable link to the media file. "False" is used when it's something else. In the absence of either the feed reader assumes "True". If you're using iWeb you don't really have access to the feed but I don't know exactly how it will behave as I've never used it for podcasting. However even if it is a link but is flagged as "False" I don't see that it matters: iTunes will still see each guid as an identifier as long as each is different.

2- I want to make some changes in previous episodes of the podcast. For instance I want to change the title of the episode. But I want to be sure that it will not be duplicated in the iTunes of the people. Can I change everything if I don't change the guid?

That's a good question and I can't be sure of the answer. Theoretically, yes, as long as you don't change the 'guid' iTunes should still think it's the same episode; but I can't guarantee it. Try it with one episode first and see what happens (upload the change having subscribed first and then update the subscription).

The 'guid' tag has to contain something which is different in every episode. By checking this the Store can tell whether a new episode has been added, though in its absence the Store will go by the URL of the media file, so it's not a disaster if it isn't there.

You could just put incrementing numbers in it: most people put the URL of the media file there. If you don't use the URL then strictly your tag should look like this:

<guid isPermaLink="false">IDENTIFIER</guid>

if you use the URL it's not essential to include the permalink, but if you do it would be

<guid isPermaLink="true">URL</guid>