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).
Instead of using the word challenging you can use the word gifted, special, special gifted.
Will, when, why, it, can
or
Myself
I have a stomach ache.
mixture
are
used to be
Answer.
is, it, in the past.
no "It" is not used instead of he and she...
Use the word "how" when asking "in what manner". Use the word "why" when asking "for what reason".