I want my subscript on F in the first equation to be like the subscript on A in the second equation.
I've tried to use \emph{ }
to override the bolding, but this makes the spacing and sizing of the subscript smaller/closer together than that for A.
\documentclass{article}
\usepackage[fleqn]{amsmath}
\begin{document}
\begin{gather}
Y_i = \bf{F}_{ij} \bf{B} + e_{i}\\
Y_i = A_{ij} = 1,2,...,N \notag
\end{gather}
\end{document}