you can use the
aligned environment from the
m &=n \\
a &=b + 1 \\
c &=d \\
e &=f \\
this is generally recommended for straightforward expressions rather than embedding a sub-structure within
align. see the user's manual for
amsldoc.pdf on ctan or type
texdoc amsmath if you have a tex live installation).
equation environment to
gather to allow multiple sub-structures, and added an example of
split as requested. since single letters on either side of the equals sign will give the illusion that everything is aligned, i lengthened one line
to show that the alignments of the two sub-structures are really offset from one another.
this result will get two equation numbers; either may be removed by use of
the positioning will not be optimal. sadly, i don't have a solution for that.