It is used when what you are saying is based on what you have heard, not on what you know is true and therefore fact.
Apparently, she resigned because she had an argument with her boss.
There is, apparently, going to be an announcement about the new CEO tomorrow.
4. As far as I know/as far as I am aware
These two expressions are used when you have partial (incomplete) knowledge of an issue or fact.
No one has complained, as far as I know.
As far as I am aware, the invitations to the party have all been sent.
5. To the best of my knowledge
This phrase is used for saying that you think something is true, but you are not completely certain. This is quite a formal expression.
当你认为某事是真的，但又不完全确定的时候可以使用这个短语。To the best of my knowledge是相当正式的表达。
To the best of my knowledge, no similar book has been published.
6. Not to my knowledge
This is used for saying that you think something is not true, although you are not completely certain.
- Has the report been sent yet?
- Not to my knowledge.
7. I imagine/suppose/guess
These are used when you think something is probably true, but you can't be sure. "Guess" is more frequently used in American English, although you can hear it in British English, too. "Suppose" is more characteristic of British English and is often used in the negative.
I imagine they've already left for the airport.
I suppose she must be delighted about getting the job.
I don't suppose you'd consider staying for another week?
I guess he will want to meet all the team members before the conference.