When mana abilities make a difference

You control a Deathrite Shaman and another creature, all your lands are tapped, but there's a land in a graveyard. Your opponent controls a Sphere of Safety (and no other enchantments.) Question: Can you attack?

If your answer was "of course, just pay the required mana cost with the Shaman", then you haven't been reading this blog enough. Naturally the correct answer is "no", but the interesting part is the reason why not.

Deathrite Shaman's first ability is not a so-called mana ability because it targets. This is defined in the rules of the game as follows:

605.1a An activated ability is a mana ability if it meets three criteria: it doesn't have a target, it could put mana into a player's mana pool when it resolves, and it's not a loyalty ability.

Since it's not a mana ability, this means that it can only be activated any time you could cast an instant, in other words, any time you have priority. The difference with a mana ability is that the latter can be used any time you have priority and whenever something requires a mana cost to be paid. (Also, mana abilities do not use the stack, while normal activated abilities do.) In the vast majority of cases this is inconsequential, but in this particular case it makes a big difference.

The cost imposed by Sphere of Safety has to be paid when declaring attackers. This is done at the beginning of the declare attackers step. No players get priority at the beginning of that step until all attackers have been declared. (Paying possible costs is part of the process of declaring attackers, as specified by the rules.) This means that only mana abilities can be used to pay the cost. Instant-speed abilities cannot.

Activating Shaman's ability in the previous step (ie. the beginning of combat step) does not help because mana pools empty between steps.

Another question: Suppose you control a creature and one untapped land that's enchanted with a Verdant Haven. Your opponent controls a Sphere of Safety and another enchantment (thus requiring you to pay 2 mana to attack with the creature.) Can you attack?

This is another interesting question because this is not an activated ability but a triggered one. Does the same principle apply to triggered abilities that does to activated abilities?

The answer is yes: A triggered ability that adds mana to your mana pool and doesn't target is also a mana ability and thus does not use the stack and will immediately add the mana whenever it triggers. Thus you can attack.