“Had I known” has very much the same meaning as “If I had known”; if there is a difference, it is in style (“Had I known” is slightly more rhetorical). “If I knew” uses a different tense to “If I had known” – the simple past rather than the past perfect.

This expression is used when we are talking about something we did not know in the past, but we wish had known it at the time. Our action in the past would have been different if we knew that information. This sentence pattern has exactly the same meaning as “If I had known that…”.

What does has-been mean? Has-been is a negative term for a person who is considered to have lost or to be far removed from the success, popularity, or skill that they once had. Has-been is used as an insult. This can be an insult or a compliment, depending on how it’s used.

• applied,
• employed,
• exercised,
• exploited,
• harnessed,
• operated,
• utilized.

/ˈhæzˌbɪn/ a person who was famous, important, admired, or good at something in the past, but is no longer any of these: She’s a has-been TV star. (Definition of has-been from the Cambridge Academic Content Dictionary © Cambridge University Press)

‘Has been’ and ‘have been’ suggest an action that started in the past, but continues in the present. When we are talking about the present: If the subject of a sentence is I – You – We – They or a plural noun (cars, birds, children) we use ‘have been’.

Auxillary verb ‘ is ‘ is invariably used in present tense while ‘ been’is applied to past tense . It is the past participle form of ‘ be’ , thus it’s always incorrect to write ‘ is been ‘ . The combination ‘ has been ‘ is correct that shows the sign of present perfect tense.

Definition of I have been there —used to say that one has experienced the same thing that someone else has experienced I know how you feel. I’ve been there myself.

Had is used as an auxiliary verb for past perfect tenses. Was is used as an auxiliary verb for past continuous tense.