Hello , dear teacher/ moderator.

Consider four sentences written below ,

1. He says that Bombay “has been” like home to his family for the past years.
2. Now, how can we be sure we “have actually been” of help?.
3. She “has been” from Italy.
4. I ‘ve taught mathematics using this book for several years.For 3-credit-hour classes , we “have been” able to omit many sections without difficulty from the book.

I do encounter the subtle differences about the usage of “has/have/had been...” and “is/are/was/were..”.The above sentences show the pattern of written examples. I think I can replace “has been” with “is”, “have been” with “are” and so forth , but I confuse with the situation.In other words, I do not understand when “has been” or “is” seems necessary. Could you please ellaborate and clear the ambiguity..
Generally, I agree with Yogi's explanation. My take:

1. If he still lives in Mumbai, then he may continue there longer, so 'has been' should be used. If he recently moved to Delhi, however, then 'was' is a clearer choice, in spite of the time duration phrase.

2. 'Were' if there is no chance or intention of helping further; 'have been' if they consider that they may offer future help with the problem.

3. Present perfect is wrong. A person is either Italian or not Italian. She is from Italy.

4. As with #s 1 and 3, 'have been' if they anticipate possible further omissions; 'were' if they think that no further omissions are possible. Additionally, 'are' could be used here if the speaker is considering the past/present/future (= timeless) ability to omit many sections each time the class is taught.
Hi mshahri,

I think that you can't change tenses in #1 and #4 because there is a time expression starting with "for".
You can say: I live in Helsinki.
but I cannot say: I live in Helsinki for five years. (wrong)!!!
You would have to say: I have lived (or have been living) in Helsinki for 5 years.

When it comes to #2 and #3 , I think you can replace present perfect with simple present , however, in #2, it would change the meaning a little bit.
The present perfect connects the past with the present so it implies that something started in the past and continues till now.

I hope that my explanation is correct and I'm sure you will get an opinion form another member.
 Mister Micawber's reply was promoted to an answer.