#### Answer

$-i$

#### Work Step by Step

Using $i^2=-1$ and the laws of exponents, the given expression, $
i^{19}
,$ is equivalent to
\begin{array}{l}\require{cancel}
i^{18}\cdot i
\\\\=
\left( i^2 \right)^{9}\cdot i
\\\\=
\left( -1 \right)^{9}\cdot i
\\\\=
-1\cdot i
\\\\=
-i
\end{array}