In Lyx, when I have a long equation in display mode I would like to display it over several lines for readability. But whenever I try to do that Lyx gives me an empty table. Is there some tex command I can insert into my equation to force a line split?
There's a lot about this in Help>Math, Long Formulas. Use, for example, Insert>Math>Eqnarray and add additional rows by pressing Ctrl-Enter, or Edit>Math>Add Line. In some cases you'll need Insert>Math>AMS multiline, and similar. This wiki page has a nice summary of these environments https://en.wikibooks.org/wiki/LaTeX/Advanced_Mathematics