once可以单独放在句首吗
过去分词作状语的用法？
过去分词作状语的用法？
一、过去分词(短语)作状语时，其逻辑主语通常就是句子的主语。过去分词(短语)作状语一般都用逗号同其它成分隔开。
1.作时间状语时，可转换为when或while等引导的从句，通常放在句首。
Seen from the top of the hill ( When it is seen from the top of the hill), the school looks like a big garden.
从山上看，这个学校像个大花园。
2.作条件状语时，可转换为once, if或unless等引导的从句，一般放在句首。
Given more attention ( If it was given more attention), the fire could have been avoided.
如果多注意一些，这场大火本来能避免的。
3.作原因状语时，可转换为because, as或since等引导的从句，多放在句首。
Encouraged ( As she was encouraged) by the teacher, the girl was very happy.
由于被老师鼓励，这个女孩儿非常高兴。
4.作让步状语时，可转换为though, although或even if引导的从句，常放在句首。
Left ( Although she was left) alone at home, Jenny didn’t feel afraid at all.
尽管被单独留在家里，但是珍妮一点都不害怕。
5.作方式或伴随状语时，常可转换为并列分句，可位于句首或句末。
Surrounded by his students, the teacher went into the lab. ( The teacher was surrounded by his students and he went into the lab.)
这个老师被学生们环绕着走进了实验室。
6.过去分词(短语)作状语时，有时为了强调，前面可带连词when, while, if, though, as if, unless等。
When given a physical examination, you should keep calm.
做体检的时候，你应该保持镇静。
once是基数词还是啥？
不是基数词，是频率副词。
adv. 一次；曾经
conj. 一旦
n. 一次，一回
I met Miquela once, briefly.
我见过蜜凯拉一次，时间很短。
Just for once he arrived on time.
只有这一次他按时到了。
The water is fine once you#39re in!
你一旦下了水，就会觉得水里挺舒适。
I#39ve only been there once.
我只去过那里一次。
She once ruled over a vast empire.
她曾统治过一个幅员辽阔的帝国。