can you tell me the difference in those 2 setences?

Never use the second one because it is wrong.

The word 'sure' can be an adjective or adverb, but not a verb.

Which one is the verb in the second sentence? If the word 'has' is the verb, it should be followed by a noun or noun phrase, not an adjective or adverb. We dont say "I have tall', right? We will only say "I have a book" or "I have two younger sisters".


"She was not yet sure if she could trust him" is in past tense. It means that she was not yet sure in the past but she is probably sure now.
"She is not yet sure if she could trust him" is in present tense. It means that until now she is still not yet sure.

2008-06-26 15:27:54 補充：
If you want to use present perfect tense, you should say
"she has never been sure if she could trust him"
It means that 她從來未...

不是 tense 的問題. Sure 是 形容詞, 它之前用 be 或其變體 is, are, am, was, were, will be, has been ...., 不用其他動詞.
所以
She was not sure 正確.
She has not sure 不正確.
有一個方法幫你記憶. 講英文的人常常掛在口邊的 I'm not sure...... 即 "我不肯定", am 和 not sure好像孿生兄弟, 總是一起用的. 這樣, 可以幫我們記著, SURE 之前一定是 BE 或其變體.