When answering another person’s comments, instead of saying just yes or no, we can respond with certain phrases that express our feelings towards the response we give.  For example:

• When we are fairly sure of our response to what someone else has said, we can use “think”:

“Will the report be finished on time?” “I think so.”
“Won’t Mary be coming with us?” “I don’t think so.”

• When we would like our response to be true, but we’re not sure of whether it will be or not, we use “hope”:

“Will the bus get here on time?” “I hope so.”
“Is it going to rain tomorrow?” “I hope not.”

• Normally, when we’re absolutely sure, we use “know” without a following word:

“Do you know who Brad Pitt is?” “Of course I know! He’s a rock star, right?”
“What are we having for dinner tonight?” “I don’t know”.

• However, to contrast think with know, we can say “know so”.

“Do you think we can go skiing tomorrow?” “I don’t think so; I know so!”

• You’ll notice that what follows the verb changes in each case.  Pay attention to this, as we cannot say “I don’t hope so” or “I think not”*.

*You may hear “I think not” colloquially, but it’s not standard.

.

Image courtesy of smarnad / FreeDigitalPhotos.net