"νομἰζω" and "θεωρὠ" are synonyms. "νομἰζω" means “to think”, "θεωρὠ"means “to consider”.
Νομίζω has an element of uncertainty, the possibility of being wrong.
Ex: Νομίζεις ότι γνωρίζεις τη ζωή? Όταν μεγαλώσεις θα αλλάξεις απόψεις. You think you know about life? When you grow up you are going to change views.
Θεωρὠ contains less doubt.
Ex: Θεωρώ ότι είσαι αξιόπιστος και τίμιος, γι αυτό θα σου δανίσω τα χρήματα. I consider you to be trustworthy and honest, therefore I will lend you the money.