i have no idea when we should add
a "that" in a sentence.

for example,
the force (that is) acting on him...
the space (that is) provided...
"that is" is usually omitted.

however, "that" sometimes cannot be omitted.
for example,
the book that costs $100 is interesting.

In addition,a sentence like "I think (that)....." is confusing too.
It is difficult for me to decide if I need to add or omit "that" all the time.

when should we omit "that" ?

push..... can anyone help ?                                                                                                                                                                                                                                                                               

does anyone know ?

我仲等緊.....