mjhjmtu

I am trying to create a new environment as follows

newenvironmentletters[1]%
 vspace-3mm
 ifnum #1=1%
 else
 beginmulticols#1
 fi
 beginenumerate[label=textbf(alph*)]
 %
 endenumerate
 ifnum #1=1%
 else
 endmulticols
 fi
 vspace-5mm
 

My question is: Why doesn't it work?!

Note: this is not a duplicate from this post, since I don't want to do the RenewDocumentEnvironment. Instead, I want to create a new environment.

If you look at the log you'll see that the first thing TeX complains about is:

! Illegal parameter number in definition of endletters.
<to be read again> 
 1
l.22 }

That's because you tried to use #1 in the end part of the environment, but you can't use the arguments passed to the begin part in the end part of an environment.

If you were to do that, then the usage of the environment would have to be
something like this:

beginletters1
 item stuff
endletters1

passing the argument to endletters too.

To do what you want you need to devise another way to pass that information to the end part. For instance, with a newif:

documentclassarticle
usepackageenumitem
usepackagemulticol

newififLettersMulticol
newenvironmentletters[1]%
 vspace-3mm
 ifnum #1=1
 else
 LettersMulticoltrue
 beginmulticols#1
 fi
 beginenumerate[label=textbf(alph*)]
%
 endenumerate
 ifLettersMulticol
 endmulticols
 fi
 vspace-5mm


begindocument

beginletters1
item Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
item tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
item quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
item consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
item cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
item proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
endletters

beginletters2
item Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
item tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
item quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
item consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
item cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
item proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
endletters

enddocument

Also, as it's been noted in the comments, it's not a good idea to add a % at the end of the line in ifnum tests (and a couple other cases too). When TeX is scanning the numbers in the ifnum test, it is expanding tokens, and will continue to do so until something that is not a number is found. For instance, the following (perhaps idiotic, but serves as demonstration) code

ifnum1=0%
 ifnum2=2
 1%
 else
 2%
 fi
 a%
else
 b%
fi

produces a, instead of b, as someone that took a quick glance at the ifnum1=0 would guess. If, however, you remove the very first %, the output is b.

Also, a missing % at the end of a ifnum#1=1 test will not appear in the output, because TeX's number scanning mechanism says that a number can be followed by an optional space, so this one is safe. To be sure, of course, one can end the scanning of the ifnum test with relax: ifnum#1=1relax.

In this particular case, the % wouldn't casuse troubles, but it's probably better to be on the safe side.