A substitute good is a good that can at least partly replace the function of another. Examples desktop and notebook, butter and jam, Starbucks and Coffeebean, Krispy Kreme and Dunkin Donuts etc
A complementary good is a good that must be used in conjunction with another. For instance, whiteboard and marker, car and petrol, sugar and tea etc
Can a good fall into both?
Of course. Consider Alain Anderton and John Sloman economic textbooks. A student can regard both as substitutes. In other word, Alain Anderton texts can be as good as the latter and vice versa. However, to some students both can be considered as complement goods. The argument is that, some information may be available in Anderton but not in Sloman. Likewise the other way is true. As such, both must be used simultaneously to facilitate learning
For a teacher like me and many others who are keen on case studies, one textbook will never be enough. As such we have to make cross-reference. In a nutshell, it’s all about personal preference and to some extent different cultures. Other examples of goods that can fall into both categories are like French fries and burger, tutor2u and economicshelp (major economics reference for students in A-Levels), corn flakes and milk, white board and LCD projector screen during presentation etc