8.79.3 Can you answer these questions?

8.79.3.1 Not loaded \selectlanguage* spanish
8.79.3.2 \newcommand vs \def for class development
8.79.3.3 Abstract doesn't show up when I run the program
8.79.3.1 Not loaded \selectlanguage* spanish

https://tex.stackexchange.com/landing/r/digest?cta=question&id=667788

I have installed the recent version of MiKTeX on OS Windows 11, last update: 12/02/2022, last week and I have a problem. It had never happened to me before. If I write this minimal working example ... 
[errors] [miktex] [languages] [miktex-update] [update]
 
asked by Sebastiano https://tex.stackexchange.com/landing/r/digest?cta=user&id=117876 Score of 2
8.79.3.2 \newcommand vs \def for class development

https://tex.stackexchange.com/landing/r/digest?cta=question&id=667589

By reading the contents of the file book.cls (the standard book class for LaTeX), I can see there are many uses of \def and \newcommand. What I don't seem to understand is why one is used in some ... 
[macros] [def]
 
asked by Diego Sejas https://tex.stackexchange.com/landing/r/digest?cta=user&id=166189 Score of 1
answered by Cicada https://tex.stackexchange.com/landing/r/digest?cta=user&id=182648 Score of 0
8.79.3.3 Abstract doesn't show up when I run the program

https://tex.stackexchange.com/landing/r/digest?cta=question&id=667373

I tried to make the abstract on TexWork, but it doesn't show up when I run the program. Although I used \maketitle, \newpage. It does not work. Could you help me point out which part that is not ... 
[revtex] [texworks] [abstract] [undefined] [latex-project]
 
asked by kim phan https://tex.stackexchange.com/landing/r/digest?cta=user&id=286132 Score of 1