Help me with the sub rule

If you’re talking about the “submerge” ability, it’s pretty much what makes subs the exception to many rules. Feel free to correct me if I’m wrong, everyone. I don’t have the rulebook memorized but I have a pretty good grasp on it:

During any combat, after your enemy has attacked at least once, you can “submerge,” which means that your sub retreats. However, you don’t necessarily retreat to another territory. You actually stay in the same sea zone, but you’re immune to any more attacks that turn. You can even do this on defence, making subs the only unit that can retreat when attacked. Turn submerged submarines on their side when you put them back on the map.

If enemy destroyers are present, you can’t submerge.

Also, I believe the final step of “Resolve Combat” is surfacing submerged submarines. I believe in the non-combat phase, you’re allowed to submerge your submarines, but if you do, beware that enemy ships can pass over you on their turn as if you aren’t there.

The only way to dodge doom when an enemy plane attacks a lone submarine is to submerge it after they miss. Subs can’t shoot at planes, so defenders can never choose planes as a casualty when submarines hit.

If anyone has anything to add or correct please do.

You’re a little bit off.

Submarines can use submerge after the attacker and defender have fired (at the end of one round of combat). When a submarine does this, it is no longer in combat but remains in the same sea zone in which it submerged. As long as it’s submerged it’s immune to attacks and can’t move, and doesn’t affect enemy ship movement; consider it to be just hiding there and not affecting anyone else.

Subs automatically resurface at the end of the noncombat move. This doesn’t trigger combat with enemy units because the combat move has already passed.

You can’t submerge except in combat, and you never remain submerged past the end of the current player’s noncombat move, and you can never use submerge to retreat to another sea zone (this was the case in the old axis and allies but no longer).

Destroyers prevent submerge.

If a sub surfaces in a sea zone with enemy units in it, nothing happens that turn - the combat phase has already passed for this turn so no more combats occur. During the next combat phase in which the player controls something involved in that sea zone, he decides whether to stay there and fight, or run away. Read your rulebook under the combat move section; if a territory starts with enemy units in it while you’re in it, then you have the option to run or fight. But this counts as a combat move, so you can’t move any more during noncombat even if you just retreated.

I thought destroyers negated opening fire, but not submerge? Or am I thinking of Pacific?

Destroyers negate both submerge and opening casualties. You’re probably thinking of Pacific.