1. Explain it simply
故意 (gùyì) means 'on purpose' or 'deliberately'. It tells the listener the action was intentional, not an accident.
Word order: Subject + 故意 + verb phrase, as in 他故意迟到了 (he was late on purpose).
It often appears with a negative verb to show someone withholds something on purpose: 她故意不接电话 (she deliberately doesn't answer the phone).
Keep 故意 in front of the verb; it cannot go to the end of the sentence or split the verb.
2. Show examples
他故意迟到了。
Tā gùyì chídào le.
He was late on purpose.
故意 sits before the verb 迟到
她故意不接我的电话。
Tā gùyì bù jiē wǒ de diànhuà.
She deliberately doesn't answer my phone calls.
故意 with a negative verb
他故意说错了答案。
Tā gùyì shuō cuò le dá'àn.
He gave the wrong answer on purpose.
the mistake was intentional
3. Share a tip
Put 故意 right before the verb - say 故意迟到, not 迟到故意, to stress that the action was intentional.