The ‘是...的 (shì...de)’ combination has the function of emphasising certain information, while it’s similar to English ‘It is (that)...’, therefore it’s often used to indicate a fact or past incidents, which have been confirmed to have happened. To make it clearer, below is a short summary of the use of the structure.
‘Main topic 是shì…information emphasized...的de’
Emphasize the content in between ‘是shì’ and ‘的de’
‘是shì’ will always be put right before the information to be emphasized
‘的de’ will always be at the end of sentence
Equivalent to ‘it is (that)...’
Often used to indicate a fact or past incidents
Main topic can be anything the sentence mainly is talking about, e.g. subject, objects, etc.
wǒ shì wǔ nián qián rènsh Lìli de
It’s 5 years ago that I knew Lily.
nǐ zuótiān shì zěnme lái jīchǎng de
It’s by what method you came to the airport yesterday?
wǒ shì zài Měiguó niàn dàxué de
It’s in the U.S. that I studied university.
zhè kuài shǒubiǎo shì sān bǎi kuài mǎi de
It’s at 300 RMB that this watch is bought at.
zhèxiē píngguǒ shì zài nǎli mǎi de
It’s at what place these apples are bought?
cài shì māma zuò de
It’s mum that made the dish.
‘Main topic…information emphasized...的de’
Yet, there’s some additional information for you, which ‘是shì’ from the structure ‘main topic 是shì…information emphasized...的de’, can actually be omitted, without distorting the sentence’s meaning.
wǒ hé Dàshān ( shì ) zuótiān yìqǐ qù de
It’s yesterday that me and Dashan went together.
zhè bēi hóngchá ( shì ) wǒ tèyì mǎi gěi nǐ de
I specially bought this red tea for you.
However, when the the sentences include too much content, without it ‘是shì’ could make it unclear which is the ‘main topic’ and which is the ‘information’. Therefore, in these situation ‘是shì’ is encouraged to be used to divide clearly between the ‘main topic’ and ‘information’
wǒ hé Xiǎohóng shàng zhōu zài kāfēitīng jiànmiàn de
Me and Xiaohong met in a cafe last week.
The above example is not clear about which information is emphasized, as it could mean any of the below:
wǒ shì hé Xiǎohóng shàng zhōu zài kāfēitīng jiànmiàn de
It’s Xiaohong that I met last week in a cafe.
wǒ hé Xiǎohóng shì shàng zhōu zài kāfēitīng jiànmiàn de
It’s last week that me and Xiaohong met in a cafe.
wǒ hé xiǎo hóng shàng zhōu shì zài kā fēi tiān jiàn miàn dí 。
It’s at a cafe that me and Xiaohong met last week.
‘Main topic 不是búshì…information emphasized...的de’
Lastly, to express negative form for the sentence structure ‘main topic 是shì…information emphasized...的de’, simply put ‘不bù’ before ‘是shì’, to make it ‘不是búshì’.
wǒmen zuótiān bú shì zuò dìtiě huí gōngsī de
It’s not by taking the tube that we came to office yesterday.
zhè ge lǐwù bú shì sòng gěi lǎoshī de
This gift is not for the teacher.