When do I use "lo"?
Why do I say "No lo creo" rather than "No creo"?
2 Answers
Your question involves transitive and intransitive sentences or transitive verbs that need a direct object. Where English would imply the direct object, Spanish chooses to show it.
Lo has a few other meanings that Jermias does not mention, like used with adjectives to form nouns. I believe that we have some previous threads discussing the uses of lo.
It's just like in English: I don't believe it, vrs. I don't believe.
No creo without a direct object (lo) is usually used to introduce a dependant clause:
No creo que puedas hacerlo.