What Specific Actions Can an Attacker Perform with a Successful 51% Attack?

With a successful 51% attack, the attacker can perform a double-spend, where they spend their coins and then use their majority hash power to reverse the transaction, effectively spending the coins twice. They can also prevent new transactions from being confirmed, essentially censoring or halting network activity.

Furthermore, they can prevent other miners from mining valid blocks, allowing them to monopolize block rewards.