let's go out for a walk.(改为同意句）
( How) (about ) going out for a walk?
I think's it's the best way to learn english.(改为否定句）
I (don't) (think) it (is) the best way to learn English.
He was so young that he could't join the army.(改为同意句）
He was (too) young (to)join the army.