This thread has set me thinking! I wonder how the commonly used phrases "I think so" or "I don't think so" are interpreted by non-native English speakers? They are usually used to express uncertainty in response to questions.
"Is this the train to London?"
"I think /don't think so." Which could be followed by "But I'm not sure / certain." Better check!
A questioner may ask "Do you think so?" to query the validity or sincerity of a speaker's statement.
The response would probably be "Yes I do".
Complicated language I'm thinking.
BTW The ability to think on one's feet is pretty useful nowadays