å tro - to believe
å synes - to be of the opinion
Jeg tror at det kommer til å regne.
I think that it's going to rain (but you won't know for certain until it starts raining)
Jeg synes at maten er god.
I think that the food is good (you have tasted the food, and you think it's nice)
You can also use å tro for faith.
Han tror på Gud. - He believes in God.