Separating some paragraphs with a blank line, and other paragraphs without a blank line?

Question

I want to be able to write blocks of paragraphs that share the same idea like this.

__1. This a a sentence about something. I continue with another sentence. With this sentence I conclude an idea, and a new paragraph is needed.

__2. I start another short paragraph here, but I did not leave a blank line between both paragraphs, because they talk about the same general idea. With this sentence I end that general idea and my paragraph.

__3. Now I am talking about some other stuff that is not related to the previous idea, but does not need a new section. I want to have a blank line between both paragraphs.

The __ is an indentation, and the paragraphs are numbered. Right now I have what is shown here; there is a blank line between all paragraphs, but I do not want that blank line between paragraph 1 and 2.

Right now I am using \setlength{\parskip}{\baselineskip} to get a new line between paragraphs, but I can't seem to find a way to not have that blank line between certain paragraphs.

Is there some easy way to achieve this?

Answer 1

Add \medskip (or \bigskip), that's all.

This a a sentence about something. I continue with another sentence. With
this sentence I conclude an idea, and a new paragraph is needed.

I start another short paragraph here, but I did not leave a blank line
between both paragraphs, because they talk about the same general idea. With
this sentence I end that general idea and my paragraph.

\medskip

Now I am talking about some other stuff that is not related to the previous
idea, but does not need a new section. I want to have a blank line between 
both paragraphs.

Don't set \parskip for this.

Answer 2

Perhaps the following might help:

\documentclass{article}
\newcommand{\specialparshook}{}% Hook for start of environment
\newcommand{\specialparsitemhook}{}% Hook for start of item inside environment
\newenvironment{specialpars}
  {\setcounter{enumi}{0}% Restart counter
   \specialparshook% Environment hook
   \renewcommand{\item}{%
     \specialparsitemhook% Item hook
     \leavevmode% Start new paragraph
     \refstepcounter{enumi}% Increment counter
     \theenumi.~% Print numbering
   }}{}

\setlength{\parskip}{\baselineskip}
\newcommand{\connectpars}{\vspace{-\parskip}}

\begin{document}
I want to be able to write blocks of paragraphs that share the same idea like this.

\begin{specialpars}
  \item\label{par1}
  This a a sentence about something. I continue with another sentence. With this sentence I conclude an idea, 
  and a new paragraph is needed.

  \connectpars

  \item\label{par2}
  I start another short paragraph here, but I did not leave a blank line between both paragraphs, 
  because they talk about the same general idea. With this sentence I end that general idea and my paragraph.

  \item
  Now I am talking about some other stuff that is not related to the previous idea, but does not need a new 
  section. I want to have a blank line between both paragraphs.
\end{specialpars}

The \rule{1em}{.4pt} is an indentation, and the paragraphs are numbered. Right now I have what is shown here; 
there is a blank line between all paragraphs, but I do not want that blank line between paragraph~\ref{par1} 
and~\ref{par2}.

Right now I am using \verb|\setlength{\parskip}{\baselineskip}| to get a new line between paragraphs, but I 
can't seem to find a way to not have that blank line between certain paragraphs.

Is there some easy way to achieve this?
\end{document}

The idea is to create a list-like environment. with every idea being set with an \item. Paragraphs (or \items) are connected with \connectpars, which jumps back vertically by \parskip.

Other hooks are provided, if needed, although they weren't used in this example. Note that it would not be possible to include an enumerate within this list since it uses enumi - the first-level counter for enumerate. Of course, that can be changed.