Stephen, Can you tell us more about XMind? How is it more powerful?

Thank goodness, an easy one!

Skipping past such technical terms as subjects and objects, we'll go straight to the mnemonics: If you can answer the question with "he," it's "who." If it needs "him," it's "whom." "Whom" and "him" both end with 'M's, you see.

Sorry, sexist language is critical for this one.

"Who went to the store with whom?"

Who went to the store? HE did.
With whom did he go with? With HIM.