VII. The omission of shì是
Shì是can be omitted in most cases. However, it may not be omitted under any one of the following conditions.
1. when the subject is a demonstrative pronoun such as zhè这or nà那;
2. when the sentence is negative;
3. when the emphasis is on a situation which has caused a fact.
A. Wǒ (shi4) zuò huǒchē lái de.
I came by train. /It was by train that I came.
Shì是may not be omitted in a negative sentence:
B. Wǒ bú shì zài Shànghǎi dǎ de diànhuà.
was not in
Shì是may not be omitted if the subject is a demonstrative pronoun such as:
C. Zhè shì zài
was bought in
Shì是may not be omitted if the emphasis is on a situation which has caused a fact.
D. Tā rén hěn hēi, shì shài de.
He is dark and that’s because of his exposure to the sun.