I think so

My friends use this term at the beginning of the sentence , e.q I think so that is not correct etc…I feel that is not the correct way, or am I wrong?

You are right. It’s not correct in that sentence.

You are right. ‘So’ in your example = ‘that is not correct’, so ‘so’ is redundant.

Is there any context which we can start with’ I think so’, I couldn’t find any, kindly correct me if Iam wrong

You can start with it, but someone else must have mentioned the referent to ‘so’ before:

A: Is it time to light the candles?
B: I think so.

Thanks, yeah I know this is possible start and end with I think so , but still I am not clear that I think so + something …is possible or not.sorry for disturbance .

A: Is it time to light the candles?
B: I think so as it is beginning to get dark.

thanks …