How do I make some of the sentences on the thank you page and the auto responder email to be single spaced?

Also how do I make some of the sentences on the thank you page and the auto responder email to be single spaced?

Like this?

Instead of this. 

You will have to add the additional space characters to your custom text, to break it to the new line instead of breaking it with the  'Enter' key. 

This can be done for the Thank You page and for the email template.