故意(gù yì) - deliberately, on purpose, intentional in Mandarin

故意 is nearly always an accusation or a denial. It sits directly before the verb, as in 他故意迟到, and it says the act was chosen rather than accidental. Chinese uses it far more readily than English uses "deliberately", so it turns up in ordinary domestic arguments, not just courtrooms.

The single most useful thing on this page is 我不是故意的. That is the standard apology for anything you did by accident, from treading on a foot to sending the wrong file. It needs 是 and 的 around 故意: 我不故意 is wrong.

As a modifier before a noun it takes 的, giving 故意的错误 for a deliberate mistake, and legal Chinese uses 故意 in fixed terms such as 故意伤害 for intentional harm. In everyday speech the adverbial use in front of a verb is far commoner.