What's the difference between
"The last two days have been unbearably hot"
&
"The last two days were unbearably hot"

I would like to say about the weather of last two days. So which one is correct?

Question

What's the difference between 
"The last two days have been unbearably hot"
&
"The last two days were unbearably hot"

I would like to say about the weather of last two days.  So which one is correct?

Answer

Both are correct, but slightly different.

“Have been”  means that something began in the past and has lasted into the present time. If it was hot and is still hot now you would use the sentence "The last two days have been unbearably hot".

If it was hot for two days and isn't as hot now you would use "The last two days were unbearably hot".

Answer

For instance, if today is Tuesday, I would use the first sentence (with present perfect) to describe Monday and Tuesday (the day which is on going on) and the second phrase (simple past) to describe Sunday and Monday. See ya!

Answer

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Answer

Hello Labiba,
the correct sentence is the first. We need to use the present perfect (Subj+has/have+past participle) in this case. There are many situations in which the Present Perfect is used. In this specific case, this tense is used to "describe an action that has not yet been finished".
Here another similar example: It has rained a lot this month.

Below there is an excellent article that would help you a lot with Present Perfect.
https://learnenglish.britishcouncil.org/english-grammar-reference/present-perfect

I hope that my explanation will help you. ;-)