How to correctly separate paragraphs with a blank line

Question

I want to end a paragraph with a blank line and start a new paragraph.

When I use

first paragraph\\
second paragraph

I get an underfull hbox error.

When I use

first paragraph\par
second paragraph

It doesn't insert a blank line.

How do I do this correctly?

Answer 1

LaTeX separates paragraphs every time you add at least one empty line between texts. Then, how paragraphs are formatted depends on you. Here's a good helpful website: How to change paragraph spacing in LaTeX.

The length name \parskip controls extra spacing between all paragraphs in the document. In standard classes, it is set to 0pt meaning no extra spacing is issued. For instance,

\setlength\parskip{6pt}

would cause LaTeX to add a distance of 6pt between all paragraphs.

If you read through the website, you will come across the name \baselineskip, which hold a distance between two baselines. Therefore, \setlength\parskip{\baselineskip} would separate paragraphs exactly by one empty line.

The macros \smallskip, \medskip, and \bigskip also add extra space in the place of issue but are meant for rather one-time usage. The amount of extra space they insert is held in three length names: \smallskipamount, \mdeskipamount and \bigskipamount. You could also use any of the names to insert \<xxx>skip between all paragraphs. For instance

\setlength\parskip{\bigskipamopunt}

would again add one empty line between all paragraphs.

Here's a small MWE to test paragraph skips:

\documentclass{article}
\usepackage{blindtext}
\setlength\parskip{\baselineskip}
\begin{document}
\Blinddocument
\end{document}

EDIT. BTW, \\ is not a correct way to separate paragraphs. Use either \par or empty lines (which are converted to \par).

For an arbitrary skip, insert \vspace{<length>} where <length> is a unit length s.a. 12pt, 1cm, 3em or 2ex etc. (*). However, this macro will work between paragraphs. In order to add skip at the beginning of a document e.g. before the title, or at the top of a new page you should use a starred version \vspace*{<length>} (thanks for pointing at details in comments).

(*) Keep in mind there are fixed and relative unit lengths, some of which are consider horizontal and the other vertical (thanks to bring this in comments). I found a couple of answers which may help you to understand it e.g. check this answer and this answer.