As a Dutch and English speaker from way back I know that 'Hij vind het zeker moeilijk' is a comment that means exactly the same as the English 'He must think it is difficult.' So in this case 'zeker' has the same meaning as 'must'.
Example : A young child in the creche is crying. In English we could say "He must want to go home." In Dutch "Hij wil zeker naar huis"....Not quite the same meaning as that offered by Duo. "Certainly" does not fit. I hope it will be corrected soon.
That also translates to Hij vindt het zeker moeilijk. There is not really a way to say this differently in Dutch.
He finds it difficult for certain and He certainly thinks it is difficult mean exactly the same thing if you ask me.
Think in the meaning of having an opinion is vinden, in most (all?) other meanings it is denken: think in the meaning of being unsure about something or thoughts, or thinking about people or things. Because of this vinden is used. Should you remove certainly from the sentence, you can use denken, in the given sentence (with certainly included) denken not really the most straight forward translation.
So there's no way to distinguish between "he certainly holds the opinion that it is difficult" and "he holds the opinion that it is certainly difficult"? Because those are definitely different things. In the first case you're certain about his opinion, and in the second he's certain about the difficulty.
Right, because to me that's pretty much the difference between "he finds it difficult for certain" and "he certainly thinks it is difficult".
Although now that I think about it, thinking something is difficult and finding it difficult aren't quite the same thing either, so that adds another layer of subtlety.