doubtless是形容词还是副词
一、基本规律
大家知道，英语中的后缀-less通常用于构成形容词，其相应的副词则是-lessly。如：
1. aimless—aimlessly
After several hours of aimless searching they were getting low on fuel. 在几个小时漫无目的的搜寻之后，他们快没燃料了。
They wandered aimlessly around the town. 他们在城里无目的地闲逛。
2. careless—carelessly
It was careless of him to leave the door unlocked. 他忘了锁门，真是粗心大意。
She was fined £100 for driving carelessly. 她因开车违章，被罚款100英镑。
3. effortless—effortlessly
I went up the steps in two effortless bounds. 我没费一点力气，两下就跳上了台阶。
There is so much information you can almost effortlessly find the facts for yourself. 资料这么多，你几乎可以毫不费力地找到事实真相。
4. endless—endlessly
There seemed to be an endless supply of food at the meeting. 会议上好像不断地有食物供应。
He was endlessly kind and patient with children. 他对孩子们总是非常好，很有耐心。
5. fearless—fearlessly
She was fearless in her attacks on public figures. 她对公众人物进行大胆攻击。
They fought fearlessly against the invading armies. 他们无畏地与侵略军作战。
6. fruitless—fruitlessly
After months of fruitless inquiry we finally discovered the truth. 经过几个月毫无进展的探索，我们终于发现了事情的真相。
Four years of negotiation ended fruitlessly last December. 历时4 年的谈判在去年 12 月无果而终。
7. harmless—harmlessly
Taken in small doses, this drug is completely harmless. 小剂量服用这种药是完全无害的。
Another missile exploded harmlessly outside the town. 又一枚导弹在城外爆炸，没有造成任何伤害。
8. hopeless—hopelessly
The economic crisis makes jobs almost impossible to find and even able pupils feel hopeless about job prospects. 经济危机使得找到工作几乎成为不可能，就连能力出众的学生都对工作前景不抱希望。
I know that if I start watching a soap opera I immediately become hopelessly addicted. 我知道，要是我一开始看肥皂剧，我很快就会无可救药地上瘾。
9. restlessly—restlessly
After listening to him for three hours the audience became restless. 在听他讲了三个小时之后，听众开始变得不耐烦了。
He paced up and down restlessly, trying to put his thoughts in order. 他坐立不安地走来走去，尽力想理清思绪。
10. selfless—selflessly
She will be remembered for her selfless devotion to the cause of the poor. 人们会记住她对扶贫事业的无私奉献。
I’ve never known anyone who cared so selflessly about children. 我从未见过如此无私地关心孩子的人。
11. thoughtless—thoughtlessly
It was thoughtless of you to forget your sister’s birthday. 你太疏忽了，连自己妹妹的生日也给忘了。
Nobody is perfect, and everyone acts thoughtlessly on occasion. 人无完人，谁都难保不偶尔疏忽一次。
12. useless—uselessly
It’s useless trying to convince her (that) she doesn’t need to lose any weight. 要让她相信她无需减肥是徒劳的。
Uselessly, he checked the same pockets he’d checked before. 他把先前检查过的那些口袋又查了一遍，毫无收获。