中间不加介词，加冠词a，例如：
1、She'd been seeing the boy for a while， but didn't want her parents to find out.她和这个男孩约会已有一段时间了，但不想让父母知道。
2、We travelled around for a while and landed up in Seattle.我们到处旅游了一段时间，最后抵达西雅图。
3、For a while， she lost the use of her limbs.好一会儿她四肢都动弹不得。
