In declarative sentences, we may omit the verb 是 when we speak about time or age. A sentence with or without 是 are equally correct.
I'll be 18 years old this year
wǒ jīnnián (shì) shíbā suì
Today is Friday
jīntiān (shì) xīngqíwǔ
However in negative sentences we must use 是.
It's not Friday
jīntiān bú shì xīngqíwǔ
Emphasizing feature of a subject
In this case, we put an adjective after 是 and add the particle 的 to the end of the sentence.
The room is clean
fángjiān shì gānjìng de
What he said is true
tā shuō de shì zhēn de
If a feature of an object is not emphasized, we won't use the construction 是...的.
房间干净 — we simply state a fact.
房间是干净的 — we emphasize the word “clean”. For example, a mother asks her daughter to clean her room, but she doesn't want to. Here, the daughter is emphasizing that the room is not dirty, so it doesn't need to be cleaned.
In this case, 是 will first be followed by a either a pronoun or a noun, and then with the possessive particle 的. Together they form a predicate that answers the question "whose".
The dictionary is mine
cídiǎn shì wǒ de
The computer belongs to my father
diànnǎo shì bàba de
Confirmation of something
To express confirmation or agreement, you must put 是 between the subject and the predicate.
It can be translated as “indeed” or “really”.
In such cases, 是 should be emphasized verbally.
Yes, I really didn't go to class today
wǒ jīntiān shì méi lái shàngkè
I won't hide it, I really ate three buns
wǒ shì chī le sān ge bāozi
Usually these sentences have the connotation an agreement or concession.
Negative sentence with 是
To make a negative sentence with 是, you should put the negative particle 不 before the verb 是.
The particle 不 before the verb 是 must be pronounced with second tone bú.
I'm not a student
wǒ bú shì xuésheng
The dictionary is not mine
cídiǎn bú shì wǒ de
There are two ways to ask a question in sentences with 是:
1) with the question particle 吗 at the end of the sentence.
Are you a student?
nǐ shì xuésheng ma
Is she your sister?
tā shì nǐ jiějie ma
2) with the interrogative question phrase 是不是 .
The word order here is the same as in the declarative sentence, but instead of 是 we say 是不是.
This is more colloquial type of question.
The particle 吗 is not used in an interrogative question phrase.