believe and believe in

"To believe" can mean "to trust." If someone tells you something and you think he or she is telling you the truth, you say "I believe you." It can also mean "to think" or "It is my opinion that ______": "I believe she's not home right now" or "I believe baseball is fun."

"To believe in" is slightly different; it can mean "to have faith in" or "to support." If someone tells you he doesn't think he can do something but you think he can, you say "I believe in you." "To believe in" can also be shorthand for "I believe in the existence of": to say "I believe in Santa Claus" is to say "I believe in the existence of Santa Claus" or "I believe Santa Claus exists."

Gramatically, "to believe" is used before an independent clause: "I believe we should make a left turn" or "I believe life is precious." "To believe in" is used before nouns or noun phrases: "I believe in the Easter Bunny" or "I believe in the right to own guns."