hi pals. could you check the sentence for me? thanks beforehand(3)

The student was craning ( craning his neck?)to copy the answers from the students sitting next to him when he got caught.

The little boy just pretended to have fallen asleep but the instance he heard Santa Claus chortling out side, he jumped out from his bed.

What do you think the sentences I made above? Thank you   :-) 

by the way, do you have any good idea to make my english sound more nature, authentic? How could I know which word/expression is used in writing and which is used in writing?

Hi,

I would use "craning his neck" for the first sentence.

And I would use "moment" instead of "instance" in the second. "Outside" is one word word, too. And I'd say "he jumped out of bed"

But I'm no teacher :)

As for your second post, you may try to look for english idiom databases on the net… Maybe?

For instance, there's http://www.phrases.org.uk/ 

And of course, read english, listen to english, as much as you can. :)

Best,

Yun 

Thinking about jumping out of things, here's something that's interesting:

http://www.drakemag.com/phpbb3//viewtopic.php?t=9039&start=0

The student was craning his neck to copy the answers from the students sitting next to him when he got caught.

The little boy just pretended to have fallen asleep, but the instant he heard Santa Claus chortling outside he jumped out from his bed.

 I would consider using a word beside "chortling."  I am a native speaker and I had to look it up to find out it ment laughing.