\documentclass[10pt,a5paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}

\usepackage{xcolor}

\newcounter{sentence}[section]
\renewcommand{\thesentence}{[\thesection,\arabic{sentence}]}

\newcommand{\R}{\mathbb{R}}
\newcommand{\words}{blah bla blha}

\newcommand{\todo}[1]{{\color{red}\large TODO: #1}}

\newcommand{\sent}[1]{\refstepcounter{sentence}#1\textsuperscript{\arabic{sentence}}}

\newcommand{\mymatrix}[4]{\begin{pmatrix}
#1 & #2 \\
#3 & #4
\end{pmatrix}}

\newcommand{\twocases}[4]{\begin{cases}
#1 & #2 \\
#3 & #4
\end{cases}}


\begin{document}

\section{First section}

\sent{A sentence before the first one}

\sent{This is a sentence.}
\label{s1}
\sent{This is another sentence.}

\todo{finish the first section}

This is a longer sentence that takes more than one line line maybe~no still it random words to show that how tilde works i dont know Theorem~10

\section{Second section}

\sent{A sentence in the second section.}
\label{sanother}

\vspace{2cm}

In sentence~\ref{s1}... \todo{what did I want to say?}

\[ \R \]

\words

\[
\mymatrix{x}{a_{2,2}}{0}{f(10)}
\]

\[
f(x)=\twocases{x^2}{\text{if }x>0}{-x^3}{\text{otherwise.}}
\]

\end{document}