Ότι or πως = that
Ότι / πως go after verbs that mean νομίζω (I think), λέω (I say), ξέρω (I know), πιστεύω (I believe) etc.
They can be used interchangeably.
Μου είπε ότι / πως θα έρθει. =He told me that he will come.
Νομίζω ότι / πως θα βρέξει αύριο. = I think that it will rain tomorrow.
Ξέρω ότι / πως έχεις δίκιο. = I know that you are right.
Που = 1) that ( because)
It goes after verbs or adverbs which indicate emotions.
Λυπάμαι που έφυγες. = I am sorry that (because) you left.
Χαίρομαι που ήρθες. = I am happy that (because) you came.
Ευτυχώς που σε είδα. = thankfully I saw you
Που =2) that, which, who
It goes after nouns or pronouns.
Αυτό που μου είπες είναι σωστό. = What you told me is right. (This that you told me...)
Ο άντρας που έφυγε είναι ο Γιώργος. = The man who left is George.