1) Which form do you prefer [best]?
JTT: No 'best' with prefer, Hela. Use 'best' with like.
Which form do you like best?
JTT:In order to determine how language works, Hela, it is not a great idea to ask for personal preferences, especially from a prescriptivist.
Do you know how to do a specific Google search? Why don'r you try to do this? Of course it's not likely that you'll get a hit for a full sentence, but if you do a limited structure search, it'll give you an idea of the numbers anyway
a) If I were/was (?) named Paul, I would be male.
Search: If I was AND THEN If I were
b) If I wasn't / weren’t (?) afraid of spiders, I would have picked it up.
Search: If I wasn't AND THEN If I weren't
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2) Is my sentence correct ?
I would not attempt to jump from an airplane unless I were sure of succeeding in opening my parachute.
JTT: Try these ones too - unless I were sure AND THEN unless I was sure
Report back to all of us, Hela. I'm interested in the results