I need to set Thunderbird to erase all emails to email servers when I download them.

I need to set Thunderbird to erase all emails on email servers when I download them.

Assuming this is a POP account, which you didn't say, but appears to be the case:

Account Server Settings > Leave messages on server setting. No need to uncheck that option, you may leave it checked so your messages are kept in the server for some time, just use the other settings under it to make the messages not remain on the server forever.

BTW, you know you repeated your post 3 times, right? :P