"When you must have teared(can we use woukd have done here)my card, something bad probably must have happened at that time". Is it correct? Sorry guys I did a mistake I wrote dobe instead of done. I don't know how to edit the title.
There are two problems. First there is no such word as "teared" (well, actually there is but it has nothing to do with tearing.) The word is "torn". Second, the the construction "must have torn" is almost certainly incorrect. The circumstances in which it might be correct are about as likely as winning the lottery. What you probably mean is "When you tore", not teared.