hello, this is the scenario:

I attack with Infernal dragon. Player B doesn´t destroy my monster.
I have a set frog on the field. End my turn. Dragon dies...

I CHOOSE not to reborn it....

Can I activate his effect by offering as a tribute a Monster with soul exchange or offering my frog? Am I off timimng? The card says I can special summon it back by fullfiling giving up one monster..

Please someone enlightnen me?

Yes, I believe you may use his effect to bring him back during your main phase, and I think that's the only time you can use his effect.

Infernal Dragon
Dragon/Effect LV4 ATK/2000 DEF/0
If this card attacks, it is destroyed and sent to the Graveyard during the End Phase. When this card is destroyed and sent to the Graveyard from the field, you can Special Summon this card by Tributing 1 monster.

You can't just reborn Infernal Dragon any time have to active is effect at the end phase of your turn if you don't chose to do so then you can't bring him back on next turn.

You are incorrect. You can activate it's recursion effect whenever he is destroyed and sent to the graveyard.

Well gee you think, I know what he ment if you can reborn Infernal Dragon if it is destoyed and you chose not to you can't reborn Infernal Dragon on a latter time unless it is back in deck a you redraw it or you used monster reborn or something.

I liked using Infernal in team duels by using soul exchange and creature swap during my 2nd main phase to use the opponents monsters to recover him :)

if infernal dragomn attacks you cannot activate soul exchange since you already conducted your battle phase.

Sorry I should have specified that I was using Creature Swap during the 2nd Main Phase, Soul Exchange when the opponent used an effect to destroy Infernal (although Ring got the ax now so not very useful anymore)

Well, first off, like stated by ryu, the whole scenario would be illegal, first off because you either attack or activate Soul Exchange, you can't have both.

Next, the timing for Infernal Dragon is exactly when he is destroyed and sent to the grave, no other time.

Moving on, if you chose to activate Soul Exchange, and somehow Infernal Dragon gets destroyed, you can use the monster selected for Soul Exchange in order to use Infernal Dragon's effect, at that time only.