Yhyjyk

Consider this small document:

documentclassarticle

usepackageblindtext

begindocument

blindtext

blindtext

beginenumerate
item%
blindtext

blindtext
endenumerate

enddocument

This has two paragraphs outside a list environment, and two paragraphs inside a list environment. I've noticed in this situation that paragraphs outside lists have their first line indented, whereas paragraphs inside lists do not have any indent:

Why does this occur? Can I restore the indentation inside a list?

EDIT: I notice that in lists, by default paragraphs are separated by blank lines. Preferably a solution should also get rid of that. (For context, I'm writing up a solution to an assignment, and nearly every paragraph is inside an enumerate or similar.)

item properties can be formatted with enumitem. Then you can use listparindent for enumerate environment:

documentclassarticle

usepackageblindtext
usepackageenumitem
setlist[enumerate]parsep=0pt

begindocument

blindtext

blindtext

 beginenumerate[listparindent=1.5em]

 item
 blindtext

 blindtext

 endenumerate

enddocument

EDIT: Thanks to @David, you can also set parsep to 0 if you want to get rid of that space.

The reason for this behaviour is that inside enumerate the length parindent is set to zero. So you can not use a simple indent as usual in normal text to get the space of parindent set.

You can simulate the ususal behaviour in normal text with the following code (See <===== for important code):

documentclassarticle

usepackageblindtext
usepackageenumitem % <===============================================
newlengthenumerateparindent % <=====================================

begindocument

setlengthenumerateparindentparindent % <=========================
blindtext

blindtext

beginenumerate
setlengthparindentenumerateparindent % <=========================
%showparindent 
%valueparindent
item%
blindtext

blindtext
endenumerate

Version 2:
beginenumerate[listparindent=enumerateparindent] % <================
item%
blindtext

blindtext
endenumerate

enddocument

and the wished result:

Please see that setlengthparindentenumerateparindent does only work in that enumerate list you placed the command inside ...

If you are already using package enumitem -- as mentioned in an comment -- you can use it to use the same indent defined in parindent with version 2 in my mwe.

The you get the resulting version 2: