祝贺 zhùhè congratulate

How to remember 祝贺: 祝 to wish + 贺 congratulate = congratulate.

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To congratulate / congratulations - 祝贺你! (zhùhè nǐ) congratulations!, 祝贺你毕业 congrats on graduating.
Slightly more formal than the spoken 恭喜 (gōngxǐ); 祝贺 shows up in written messages and speeches, 恭喜 in casual speech (and 恭喜发财 at New Year).
祝 alone is 'to wish' (祝你好运 wish you luck) - 祝贺 specifically means congratulating on something achieved.