8. He got down to write the composition after lunch.

8. He got down to write the composition after lunch.
He got down to writing the composition after lunch.
原因:get down to doing sth.其中的to为介词,后跟STH.或者DOING STH.,着手开始做某事

10. They left the restaurant as possible as they could after dinner.
改成:They left the restaurant as QUICKLY as they could after dinner.
或者:They left the restaurant as QUICKLY as POSSIBLE after dinner.
原因:as QUICKLY as ONE CAN =as QUICKLY as POSSIBLE .尽可能快做某事.

Corrected:

8. He sit down to write the composition after lunch.
10. They left the restaurant as soon as they could after dinner.
Please explain

Sit: Sit down to write something. Not get down to write. Get down means 爬下.

As soon as... is a phrase to use.

Bob (in US)