I sometimes get confused between the two.
Just to remmber the rigid rule, ie:

I am
You/We/They are
He/She/It is

in order to fulfil the so-called correct English grammar requirement.
I'm intrigued, what do you mean by 'so-called correct' grammar?
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"1.Two plus five is seven.
2.Two plus five are seven. "...........Which one of the sentences is correct?