"DO", in a positive sentence, is used for emphasis. It adds no meaning to the sentence - only makes it much stronger. consider this conversation: A: "I believe in God" B: "I believe in God, too" A: "Are you sure? You're just saying that...." B: "I *do* believe in God! Honest!" "do" can be used with any verb. When you use it, it often gives the sentence a feeling of "on the contrary" "I do (on the contrary!) go to church some times" "On the contrary, i *do* like brazilian food!" "I really do like watching TV" it can even be used by itself: Q:"do you like pancakes?" A: "I do (like them)" "I really do"

My asnwer follow Mr. Iangreen's logic, but I interpreted it differently. " I do belive" is the ESL speakers' English. "I believe it" is the native speakers' English. In Grammar, "I do believe" is possitive, and " I do not believe" is negative. Thus the word " Do" may goes either way, meanings: yes or no. However, "I belive it" is only possitve, you can not change it's meaning to negative by adding "not" to the statement. So, "I belive it" means possitive with no other alternative choice. Let discuss Mr. Iangreen's example: A: "I believe in God" B: "I believe in God, too" A: "Are you sure? You're just saying that...." B: "I *do* believe in God! Honest!" A: "Are you sure? You're just saying that...." and, B: "I *do* believe in God! Honest!". Here, A (native speaker) assumed that B's (ESL speaker) don't understand that " I believe" is a confirmation form of an agreement. B's responding was implying that he is also a native English Spearker, he understanding what he said. He reconfirmed, the choice between "do" and "do not", he will choose "do". In fact he chose "do" to reconfirm his statement. If the native English speaker said " I do belive" , it is more emphasis than " I belive". If the ESL speaker said " I do belive", it would be better to ask him/her " Are you sure?" It implies " do you understand ?".

I do believe puts more emphasis on that sentence. It makes it so that it stands out more. It's the same as saying do not instead of don't. -I believe that is my duck. -I do believe that is my duck. -I don't actually have a duck. -I do not actually have a duck.

I suppose that when you say "I DO believe in this" is that you want to emphasize that you really believe in something.

"I do believe" is more an answer. - Johny we had nothing. - I do believe you honey. "I believe" more like saying smth. I believe that ducks don't fly!