Which one is past tense?

They are all past tenses.

The first is the simple past.
The second is the past perfect.
The third is the present perfect (a past tense).
The fourth is the past progressive (also called the past continuous)

EnglishmavenThe third is the present perfect (a past tense).

I disagree. You may say that the state described by From that day onwards, they have been inseparable is 'anchored' in the past but still present now whereas you can't say that about the rest of sentences which refer to the actions/states buried in the past.

EnglishmavenThe third is the present perfect (a past tense).

One thing you can't call the present perfect is a past tense.

EnglishmavenThe third is the present perfect (a past tense).

I was about to say that this is debatable, but I see that previous posts have already taken up that point.

Perhaps we could say it's a tense that says something about the past but from a present point of view.

CJ

The present form of "HAVE" in combination with the past participle is called the present perfect. That is the name of the tense. It is a past tense, referring to something that occurred in the past up to the present moment. Even though it contains inclusion of the present moment, its name, as a tense, is the present perfect. It is considered a past tense. Emphasis is more on the past portion of it

Englishmaven It is a past tense, referring to something that occurred in the past up to the present moment.

Most writers consider it a present tense.

EnglishmavenEven though it contains inclusion of the present moment, its name, as a tense, is the present perfect. It is considered a past tense.

By whom?

So far as I know, everyone, but you. It is a past tense.

Check any grammar site. I have this from one:

"We use th

EnglishmavenSo far as I know, everyone, but you.

Actually, there are a lot of charts available for the tenses which don't classify "present perfect" with the past tenses.