1. shì (是) firstly can be used to join two noun phrases to indicate an equational relationship between them, in this case, the two noun phrases can be interchanged without changing the meaning of the expression.
zhè běn shū de zuò zhě shì sān máo.
sān máo shì zhè běn shū de zuò zhě.
The author of the book is San Mao.
2. When functioning as a emphasizing marker, the verb shì (是) is placed right in front of the constituent to be emphasized, but actually the information conveyed by the latter clause is the communicative focus.
tā zuó tiān shì qù liǎo xué xiào , dàn shì méi néng jiàn dào zhāng lǎo shī.
He did go to the school yesterday,but didn’t see Mr.Zhang.
3. The structure shì…de (是…的) is a pseudo-cleft construction, which is a type of construction in which the focus between the verb shì (是) and de (的) appears in the form of a relative clause. The construction has the effect of emphasizing the certain part of the sentence motivated by communication needs in a given discourses.
wǒ shì lái zhǎo nǐ de , bú shì lái kàn diàn yǐng de.
I'm here to look for you, not to see a movie.