наверно 1. (parenthesis) probably, most likely 2. (adverb) for sure, certainly, for a certainty наверное 1. = наверно 1) So, it's ok to say 'Я это знаю наверно' which means 'I know it for certain'. But 'Я это знаю наверное' is grammatically incorrect. HTH

My dictionalry tells: наверно 1. ; ( по всей вероятности) probably, most likely 2. ; ( несомненно) for sure, certainly, exactly, definitely; without fail ( =наверняка) наверное = наверно 1. But really the word "наверно" is seldom used in 2nd meaning. Only in book language, maybe, In the spoken language they usual use "наверняка".

There's nothing else to say about your question apart from the fact that they are absolutely the same and are used in the same way. I have no idea what people are talking about; they are the same and this is it! You use them when you're not sure about something of if something will happen. I don't know why we can't simply use just one of them instead of having both but we do have the two words which mean exactly the same and sound almost the same. In the following sentences there is no any difference whatever! - Я, НАВЕРНО, не смогу приехать к тебе. - Я, НАВЕРНОЕ, не смогу приехать к тебе.

I think Наверно - common language, short answer - Ты пойдешь вечером в кино? - Наверно... Наверное - literary language or long sentense - Ты пойдешь вечером в кино? - Наверное пойду... Наверное, сегодня в кино покажут что-нибудь интересное. Наверное, он сошел с ума.