These words all describe somebody who knows without doubt that something is true or will happen.
sure[not before noun] without any doubt that you are right, that something is true, that you will get something or that something will happen:
‘Is that John over there?’ ‘I’m not sure.’“那边那个人是约翰吗？” “我说不准。”
Are you sure about that?这事你肯定吗？
England must win this game to be sure of qualifying.英格兰队必须拿下这场比赛才能确保获得资格。
Sure is often used in negative statements and questions, because there is some doubt or worry over the matter. If there is no doubt, people often say quite sure: I’m quite sure (that) I left my bag here (= I have no doubt about it).
confidentcompletely sure that something will happen in the way that you want or expect:
I’m quite confident that you’ll get the job.我肯定你能得到那份工作。
The team feels confident of winning.这个队觉得有把握取胜。
Confident is a stronger and more definite word than sure and is more often used in positive statements, when you feel no worry.
convinced[not before noun] completely sure that something is true or right, especially because the evidence seems to prove it or somebody else has persuaded you to believe it:
I’m convinced that she’s innocent.我坚信她是清白无辜的。
certain[not usually before noun] sure that you are right or that something is true:
Are you absolutely certain about this?你对这事绝对确信无疑吗？
sure or certain?用 sure 还是 certain？Like sure, certain is often used in negative statements and questions. It is slightly more formal than sure; sure is more frequent, especially in spoken English.
positive[not before noun] (rather informal) completely sure that something is true:
She was positive that he’d been there.她确信他曾到过那儿。
‘Are you sure?’ ‘Positive.’“你敢肯定吗？”“绝对肯定。”
clear(often used in negative statements and questions) having no doubt or confusion about something:
My memory isn’t really clear on that point.那一点我记不太清楚了。
Patterns
sure/confident/convinced/certain/positive/clear about something