When the occurrence of something is known,“是……的" can be used to emphasize the time, manner, place, purpose and doer involved in the occurrence of the event. “是”can be omitted in positive and interrogative sentences, but not in negative sentences. tā shì jīn tiān lái běi jīnɡ de 。 她是今天来北京的。She came to Beijing today. wǒ shì kāi chē qù xī ān de 。 我是开车去西安的。I went to Xi 'an by car. zhè shì zài shànɡ hǎi mǎi de 。 这是在上海买的。I bought it in Shanghai. wǒ shì lái yún nán lǚ yóu de 。 我是来云南旅游的。I came to Yunnan for traveling. wǎn fàn shì dà wèi zuò de 。 晚饭是大卫做的。David cooked dinner. wǒ men bú shì zuò fēi jī qù de 。 我们不是坐飞机去的。We didn't go by plane.