By default,
\begin{enumerate}
\item pif
\begin{enumerate}
\item i
\item j
\item k
\end{enumerate}
\item paf
\item pouf
\end{enumerate}
produces
1. pif
a) i
b) j
c) k
2. paf
3. pouf
instead of
1. pif
1.1 i
1.2 j
1.3 k
2. paf
3. pouf
But fortunately, the enumitem package is here, and
\usepackage{enumitem}
\begin{document}
\begin{enumerate}[label\*=\\arabic\*.]
\item pif
\begin{enumerate}[label\*=\\arabic\*.]
\item i
\item j
\item k
\end{enumerate}
\item paf
\item pouf
\end{enumerate}
\end{document}
will produce
1. pif
1.1 i
1.2 j
1.3 k
2. paf
3. pouf
This was the random LaTeX achievement of the day.