So here I'm going to be talking about the differences between effects that state When x... you can Y, IF X... you can Y, and When OR If : Y

So an example for each:

When x... you can Y:
Elemental Hero: The Shining

His effect states; When this card if sent from the field to the graveyard, you can add up to 2 banished Elemental Hero monsters from your banish pile to your hand.

If X... you can Y;
Doppel Warrior

His effect states; If this card is sent to the graveyard as a synchro material monster, you can special summon 2 doppel tokens...

When OR If : Y
Sangan

His effect states; When this card is sent from the field to the graveyard: add 1 monster with 1500- attack from your deck to your hand

So all of these are very different for purposes on whether or not it misses timing, and whether or not its optional.
Missing timing:
A monster that says When X you can Y, WILL miss timing if the "When" part of the effect is not the final piece of the chain to resolve. For example, if EH Shining is torrential'd, he activates. The final in the chain is him being destroyed. However, if EH Shining is used for a tribute summon of say, Caius the Shadow Monarch, His effect would NOT activate. In this situation EHS would first be sent to grave, and then caius summoned. Since the final thing to happen is caius being summoned EHS misses timing. His "when" is passed and gone already.
A monster who says When X you can Y misses timing if used in a synchro summon or tributed for a tribute summon. It also misses timing if for ANY OTHER REASON the "when" is not the final piece of the chain to resolve(ie the chainlink1) When, Can effects are optional, if the situation allows the effect to be activated it is your choice whether or not to activate.

A Monster that says IF x you can y, cannot miss timing under any circumstances. This is because it doesnt say "when". When doesnt activate because right at the time its effect would be allowed to activate, other stuff is still going on and by the time that stuff is over the window to activate is over. an IF effect, however, does not have to activate right when the window hits. For example, if Doppel warrior is used in a synchro summon, his effect activates. this is because doppel can wait until the current event is over to activate his effect. Even though he was sent to grave before the synchro summon, The situation still happened, and that is all doppel needs to activate. an IF, Can effect can be activated on choice when the situation occurs and will NEVER miss timing.

A monster that says When X: Y, also cannot miss timing under any circumstances. This is because the effect is MANDITORY(that means you have to do it). It is not a choice. You MUST activate the effect. Given that you dont have a choice, it wont miss timing. This also means, however, that you cannot choose not to activate. so if your sangan is destroyed you MUST add a 1500- monster from deck to hand(if you have any). Manditory effects can still be negated by cards like Thunder King Rai-Oh, of course.


-Chees

cool topic , well really help to start to understand timing