Hi HazelLees01
welcome to the Apple (user-to-user) Community.
Re: how to select multiple lines of text using trackpad
To select multiple lines of text in the Mail app:
with the cursor at the point you want to begin highlighting, press on the trackpad and drag down (and/ or across) so that all the specific text you want to copy is highlighted
while the lines are highlighted, choose Edit > Copy from the to top menu bar
(or use the keyboard shortcut : press Command and C to copy)
Then with cursor in the place you want:
you can choose Edit > Paste (or use the keyboard shortcut : Press Command and V to paste)
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Re: When I right click on the highlighted lines it deselects all the lines. What setting do I need to change?
See Apple support's online article: How to: Change Track pad settings on Mac - Apple Support
All the best :-)