I
adjective
BAD : I don't have any grave problems.
GOOD : I don't have any serious problems.
BAD : Blackmail is a grave crime.
GOOD : Blackmail is a serious crime.
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grave = extremely serious and worrying, especially because someone's life or happiness is in danger: 'The situation has become increasingly grave and war now seems inevitable.' 'The shortage of food and medical supplies is giving rise to grave concern.'
II
noun
BAD : We went to see the 'Valley of the Kings', where the grave of Tut-mah-Amun was found.
GOOD : We went to see the 'Valley of the Kings', where the tomb of Tut-mah-Amun was found.
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grave = a place in the ground where a dead person is put: 'The coffin was slowly lowered into the grave.'
tomb (pronounced /tu:m/) = a place where a dead person is put. A tomb is usually made of stone and is above the ground: 'Our first stop was the Tomb of the Unknown Soldier.'