Błędnie: My great grandfather tends to be malicious and grumpy. He's ery old though, and we're not allowed to behave tactlessly when he's around. Poprawnie: My great-grandfather tends to be malicious and grumpy. He's ery old though, and we're not allowed to behave tactlessly when he's around.
"Great-grandfather" to 'pradziadek'. "Great grandfather" to związek rzeczownikowy oznaczający 'wspaniałego dziadka'.