Is it true to say 51% of nodes should validate a transaction? I've read some articles about the bitcoin network, some of them say it is necessary for a transaction, before adding to blockchain, to be validated by more than half of all nodes in bitcoin network. Is my understanding true? If yes, is it true to say that after exactly 51% of nodes have validated the transaction, it will be added to blockchain?
No, it is not necessary for any node validate a transaction before it is included in a block and thus added to the blockchain. A transaction can be included in a block without being validated, but doing so risks the transaction being invalid and thus making the block invalid. This has happened several times in the past; miners will make their own transaction and include it in their own block without broadcasting it to the rest of the network.