To be frank, the phrase "坦白说" would be translated to "to be frank" in English. This phrase is used to indicate that the speaker is about to express their honest thoughts or opinions on a matter.

For example:

"To be frank, I don't think that plan will work." (坦白说，我认为那个计划不会奏效。)

"Can I be frank with you for a minute?" (我能坦白跟你说一分钟吗？)

The phrase "to be frank" is derived from the word "frank," which means "direct and open in speech or writing; sincere." It is often used as a way to introduce an honest and candid opinion or statement.