"是……的" is a special sentence structure to emphasize or clarifying part of the detail information. It is used very often in everyday Chinese. One thing worth noting is that this sentence structure is mainly used when the action took place in the past.
The basic rule of how to transfer a normal sentence into a 是……的 sentence structure is: first, find the part that you want to clarify or emphasize, then add 是 before that part and add 的 at the end of the sentence.
Let's get better understanding of this sentence structure through some examples from simple ones to complex ones.
He went to Xi'an yesterday.
Notice that 了 is used to imply the action already completed in the past.
If you want to emphasize that he went to Xi'an yesterday, not another day. Then the time part of the sentence should be emphasize, so we add 是 in front of 昨天, and add 的 at the end of the sentence. Also, please note that you need to drop the 了 in the original sentence.
(The meaning more like: It's yesterday when he went to Xi'an)
Now I am adding more information into the original sentence:
He went to Xi'an by plane yesterday.
To emphasize the time part:
Maybe you want to clarify that he took plane, not train or bus. Then you need to add 是 before the way of transportation and add 的 at the end of the sentence.
Again I am adding more information into the original sentence:
He went to Xi'an by plane yesterday to visit a friend.
To clarify the time:
To emphasize the way of transportation:
To emphasize the purpose:
Now it's your turn. Try to do the following exercise. Type your answer as comments to get feedback from me. :)
She went to Britain to learn English.
1. Write your sentence with 是…… 的 structure to emphasize the place is Britain not anywhere else.
2. Write your sentence with 是…… 的 structure to clarify that her purpose is to learn English not any subject else.
How to make negative sentence with 是……的 structure
Making question with 是……的structure
More complete information about this sentence structure can be found at Chinese Grammar Wiki.