在你的句子中，不能说“后面的句子是修饰前面的 it”，因为逗号后面是一个时间状语从句，要说修饰的话，应该是说它修饰主句的谓语动词model。至于这个 it 的指代问题，我在网上查了一下你的句子，查到了它的出处（注意此句的上下文以及 after 要小写；同时原文中用的是反身代词 itself）： 

What do you think of your Chinese? Is it good or bad? If you think you are good at Chinese, You can take part in the competition named the Chinese Characters Dictation Competition (汉字听写大赛). The competition became popular among the teenagers. Let’s know something about it.

More and more computers and smart phones are used by teenagers. They spend too much time on the Internet in playing or chatting. They are used to sending messages by computers and smart phones. Some of them forget to write Chinese Characters by hand. The teachers and parents are very worried about it. What can they do?

Not long ago, they found an interesting game called the Chinese characters Dictation. The program modeled itself after the American show “National Spelling Bee”, where middle school students stand on the stage to write down Chinese via (通过) dictation. Over two hundred competitiors are from forty-six schools. The audience (观众) are interested in this new kind of competition, too. They would like to write down the characters while the competitors were writing them. But they found it was also difficult for them to write down all the characters.

Is it an interesting competition? Can you be the winner?

注意：原句用的是 itself，model oneself after 是习语，意思是“模仿……”。如：

She models herself after her mother. 她模仿她的母亲。（张道真《现代英语用法词典》）

The program modeled itself after the American show “National Spelling Bee” 的意思是：这个节目模仿的是美国拼字大赛节目。