Remember that you can use 是 and 的 if you want to emphasize certain parts of a sentence. For example, 我们是三天前到的.
You can also use 是 and 的 to emphasize an adjective, like the difference between saying She is serious and She is SERIOUS.
她是认真的！
Tā shì rènzhēn de!
She is serious!
那是好的！
Nà shì hǎo de!
That is good!
这不是真的！
Zhè búshì zhēn de!
This is not real!