The words previous and last can in many cases be used interchangeably. For example, I can refer to the week before the current week either as
the previous week.
However, the word last also has a different meaning, namely that of final or ultimate. If I were to change jobs, I could say
This is my last week.
It is important to keep the second meaning in mind because it can lead to confusion and ambiguity. A good example, often encountered in scientific writing, is the following. When you refer to the previous chapter in a text or book, the expression
in the last chapter
is ambiguous because it can also be interpreted as referring to the final chapter of the book. Therefore, you should either write
in the previous chapter
or, even better,
In chapter x,
where x is the number of the chapter you are referring to.
Intererestingly, the same problem also exists in German, where the phrase
im letzten Kapitel
can either mean in the previous chapter or in the final chapter.