Here you need a past participial adjective, the third form of a verb. "used" is correct in your example.
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Thinking SpainThanks for your help, but I have never heard of past participal adjectives. Everything learnt, everything to be learn!So do I' is an informal expression, which is used in conversations to express the same opinion or idea.
One more, can I assume that after 'be' I always have to use 'used'?
Thinking SpainThanks! Yes it does.Yes.
Then, can I assume that after 'be' I always have to use 'used'?