The's no single word that you can use to replace is. It depends on the context.
Examples
The book is lying on the table (for The book is on the table).
The castle stands at the top of the mountain (for The castle is at the top of the mountain).
You can use the word "demanding" as a synonym for "challenging."
Instead of using I in a essay paper what word do you use?
Will, when, why, it, can
or
mixture
I have a stomach ache.
Myself
are
no "It" is not used instead of he and she...
used to be
is, it, in the past.
Answer.