反倒: on the contrary, instead (colloquial)
【高34】【高107】语气副词、复句 / adverbs
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
反倒 ("on the contrary, instead") signals a result opposite to what was expected or hoped for: 吃了药，他的病反倒更重了。 It goes after the subject, before the verb or adjective. To put it before the subject, lean it on 是: 反倒是他先道歉了。
It is used above all for a surprised reproach (明明是你的错，怎么反倒怪我了？) or an unexpected reaction (听护士这么一说，他反倒放松了下来). The context sets up the expectation, often with 不但没…… or 越是……反倒越……
反倒 is the spoken variant of 反而 (HSK 3 and 4), which is more neutral and commoner in writing; it contains 倒 (HSK 5), which already marks an unexpected contrast. It needs a real contrast: if the result was only to be expected, say 所以 or 就.
Examples
吃了药，他的病反倒更重了。
Chī le yào, tā de bìng fǎndào gèng zhòng le.He took the medicine, and his illness got worse instead.
她没有生气，反倒笑了起来。
Tā méiyǒu shēngqì, fǎndào xiào le qǐlái.She didn't get angry; on the contrary, she burst out laughing.
我好心提醒他，他反倒不高兴了。
Wǒ hǎoxīn tíxǐng tā, tā fǎndào bù gāoxìng le.I warned him out of kindness, and he got upset instead.
明明是你的错，怎么反倒怪我了？
Míngmíng shì nǐ de cuò, zěnme fǎndào guài wǒ le?It was clearly your fault, so how come you're blaming me instead?
听护士这么一说，他反倒放松了下来。
Tīng hùshi zhème yì shuō, tā fǎndào fàngsōng le xiàlái.After hearing what the nurse said, he actually relaxed.
他越是在困难的时候，反倒越能坚持。
Tā yuè shì zài kùnnan de shíhou, fǎndào yuè néng jiānchí.The harder the times, the better he is at holding on.
Common mistakes
✗ 我很累，反倒想睡觉。 → ✓ 我很累，所以想睡觉。 (nothing runs against expectation, so use 所以).
✗ 反倒他帮了我。 → ✓ 他反倒帮了我。 / 反倒是他帮了我。 (as a rule 反倒 follows the subject; before the subject it needs 是).