Я знаю это наверное. = I know this for sure. (old Russian)
Наверно(е), я это знаю. = Probably, I know it. (modern Russian)

Stresses: навЕрно, навЕрное

Actually, наверное literally means "for sure". The same roots etc. But I don't use "наверное" as "probably". Well, I try to keep myself from it, to be honest. But I'm a bit old fashioned person, so I can't recommend you to copy my language behavior otherwise you would sound like a hero of Pushkin's poems.
Also, "for sure", "certainly" = наверняка (in modern Russian). Sometimes this word may help you to avoid misunderstanding.

Stress: навернякА