how to delete movies from iphone, but keep them in itune in my PC??

All iTunes content on your iPhone - all music, movies, TV shows and 3rd party apps on your iPhone should be in your iTunes library on your computer. You can delete a movie, TV show, and an app direct from your iPhone but this will have no effect on your iTunes library. If a movie, TV show, or 3rd party app that is deleted direct from your iPhone remains selected to be transferred from your iTunes library on your computer to your iPhone, the content will be re-transferred to your iPhone with the next sync.

What is selected under the Movies tab and TV Shows tab for your iPhone sync preferences with iTunes? If Sync Movies and Sync TV Shows is selected - you aren't manually managing music and videos with your iPhone, you can deselect a movie under the Movies tab and deselect a TV show under the TV Shows tab for your iPhone sync preferences with iTunes that you want removed from your iPhone followed by a sync.

iTunes does not delete media from the iTunes library just because it is not on the iphone.

Backing up your media on an external drive is a good idea.
You can just grab the entire iTunes folder and drag it to the new drive and drop it, copying it over.

Message was edited by: Johnathan Burger

This is correct.

If you have Sync Movies and Sync TV Shows selected, deselecting this followed by a sync will remove all movies and TV shows from your iPhone, and the same applies when having Sync Music selected.

Under the TV Shows tab for your iPhone sync preferences, select automatically include ALL episodes of selected shows. Select the TV shows in the window below you want transferred to your iPhone followed by a sync. To remove a show later, deselect the show under the TV Shows tab followed by a sync.