question for linguists

[Emily Tall]

Can someone explain to me when you need to use "to, chto" to join 2
clauses and when you should omit it? (I am concerned only with use in
the accusative case.) For example, to translate "I want to understand
what I do," a student wrote: Ja xochu ponimat' to, chto ja delaju.
Is there any difference in meaning of the "to" is omitted? Some of
my students learned about "to,chto" while studying abroad and now
they are inserting it everywhere, but I don't know how to explain when
not to use it. I've checked in numerous textbooks and reference works.
Thanks! Emily Tall    mllemily at ubvms.cc.buffalo.edu