"We think otherwise."
Translation:Ni pensas alie.
I wonder if pensi is really the word you want to use here. Pensi is more of an active (not gramatically) verb, meaning to think about, to ponder. This sentence would mean that we use a different thought process, not that we come to a different conclusion (which is more the sense you get from the English sentence). If you were translating "We think otherwise" into Esperanto, you would probably use kredi or opinii.
Maybe that's too picky, but I don't think you want to get the wrong sense of pensi in people's heads, as if it were a perfect overlap with the English word think.
For those who insist on "opinii" rather than "pensi" I say this: First we have to think, and after thinking we may draw a conclusion and then we can opine. (yes the word exists in English) So whether you use "pensi" or "opinii" just shows which end of the process you want to stress. But I reluctantly admit that there are people who will opine without thinking:)
From Jordan's "Being Colloquial in Esperanto":
pensi = to think, consider, ponder. Not to be confused with: kredi = to believe opinii = to have the opinion that.
No, "ali-" is not a correlative.
Some people use alie for "elsewhere", aliel for "otherwise, differently", alies for "someone else's"... but I don't think anyone takes it all the way and uses aliu adjectivally ("Kiun libron vi volas? Ne tiun libron sed aliun libron") -- there, they use alia even though if it were a correlative that would mean "a different kind of book" rather than "a different book". Nor have I seen alio for standard alia afero "something else".