So, I was trying to go through "Nova methodus pro maximis et minimis, itemque tangentibus,…, & singulare pro illis calculi genus". However the paper is in a language I don't know (Latin maybe). This is one of the earliest published works on the modern calculus.
Is there an English translation of this great work available?
On
The wikipedia article on the paper, https://en.m.wikipedia.org/wiki/Nova_Methodus_pro_Maximis_et_Minimis, contains a link to this PDF file which seems to be exactly what you want: http://17centurymaths.com/contents/Leibniz/nova1.pdf
A good source for this sort of stuff is David Eugene Smith's book A source book in mathematics. It has collected a bunch of important mathematical papers through history. In particular, there is a partial translation of that Leibniz's paper by Evelyn Walker (under the title "Leibniz on the Calculus", at page 619 in the copy I have (a Dover edition)).
An online copy of the book (1929 edition) is available here.