Learn Croatian: Dabogda propao tvoj plan!

This sentence uses the optative mood to express a strong wish or curse. You form it by using the bare l-participle (the form usually used to build the past tense) all by itself, completely dropping the auxiliary verb (je). Here, propao expresses the wish "may it fail!".

What is the literal meaning of dabogda?

Dabogda is a particle used for strong wishes, most commonly curses. It is actually a fusion of the phrase dao Bog da, which literally translates to "may God grant that" or "God gave that". Even though it mentions God, it is widely used in everyday language simply to mean a strong "may it happen that" or "I hope" for something bad.

Could I use the imperfective verb propadati instead of the perfective propasti (propao) here?

Not naturally. When making wishes or curses with the optative mood, you almost always use perfective verbs because you are wishing for a completed, definitive outcome. You want the plan to completely fail (propasti), not to just be in the ongoing process of failing (propadati).

Why is the subject tvoj plan at the very end of the sentence?