Learn Italian: Il Suo sogno non si avvera mai. Lo deve capire. - Your dream never comes true. You must understand it.

Here, Lo is acting as a pro-clausal pronoun. Instead of replacing a specific masculine noun, it replaces the entire preceding idea: Il Suo sogno non si avvera mai. It essentially translates to "this fact" or "it" in the phrase "You must understand it."

Could I attach Lo to the end of capire instead?

Yes, absolutely! Because deve is a modal verb paired with an infinitive, you have two choices for placing the pronoun. You can put it before the conjugated verb (Lo deve capire) or attach it to the end of the infinitive after dropping the final -e (Deve capirlo). Both mean exactly the same thing.

Why is Suo capitalized, and who is the speaker addressing?

The speaker is addressing someone formally using the Lei (formal "you") form. The capitalized Suo means "Your" (formal), and the verb deve is in the third-person singular to match this formal address. If this were a casual conversation with a friend, it would be Il tuo sogno and Lo devi capire.

Why is there a si before avvera?