I'm a bit confused by how opposed/parallel/defensive actions are supposed to work.

Page 44 says:

The DEFENSIVE ACTION is a special action
available during the negotiation phase. It is
always made with a PASSIVE ABILITY (one of
ENDURE (V), REACT (I) and RESIST (R)) and it’s
always an opposed action for bonus dice.
Furthermore, it’s the only opposed action for
bonus dice — other Abilities may only
generate bonus dice with parallel actions!

Each pair of actions
is either OPPOSED or PARALLEL

If the players keep switching their
actions around because they want different
action types, the Story Guide helps them
resolve the disagreement. Ultimately the
player who wants opposed actions will need
to make a DEFENSIVE ACTION or back down.

The confusion resides in the fact that the defensive action is formally defined as an opposed action for bonus dice - that is, the only opposed actions for bonus dice in the system are defensive actions, which must be made with Passive Abilities. You can't make a defensive action without another action that you're opposing, because if there is no such action, then there's nothing to defend against. As for why only defensive actions can be opposed for bonus dice, that's a quirk, simply enough: it's just a little something to give flavour and asymmetry to the proceedings. It's also notable that opposed for bonus dice is perhaps the most static action type in the game, so making it so that only defensive actions can be use that combination gives it a bit more flavour.

Thusly, to answer your queries:

• A single action can be opposed in the sense that it is opposed against some other action. That is, you can't make a defensive action unless you also specify the action you're opposing. If there is no opposing action, you can't defend.• What page 43 means is that if one player wants parallel and the other wants opposed, the guy who wants opposed can have his way, but only if he's willing to turn his opposed action into a defensive action. Defensive actions are opposed actions, after all; the point here is that you can always defend yourself by system fiat (there is no way to get an undefendable attack just by refusing constructive dialogue), but your right to defense does not actually extend to dictating any opposed action you want: you can only insist on a defensive action, you can't force the opponent into opposed action with other Abilities aside from the Passive ones.• Your example about parallel for bonus dice vs. defensive action is basically correct: the parallel for bonus dice has to change into opposed for Harm for the defensive action to make sense. Note, however, that the burden of proof is on the player who is insisting that he'll "defend against" the parallel bonus dice action! As parallel bonus dice actions are usually somehow secondary and non-aggressive, it might be the case that the defensive player can't really justify the defensive action in the fiction, at least not if the other player doesn't want to cooperate and be a little bit flexible. So in practice you can only have a defensive action if you can actually phrase the fictional events in such a way that the character actually defends against something - and if he does defend, then it makes sense that a failure of that defense will cause Harm.• Your comparison of defensive actions vs. parallel actions for bonus dice is fair - there rarely are situations where you'll benefit by foregoing defense and just generating bonus dice. Such situations do exist, though: if somebody else is engaging the opponent so you can securely generate bonus dice, then you might wish to do so. Likewise, if you're gearing up to an action that can't be allowed to fail, you might wish to sacrifice your well-being (likely going to take a big Harm if the opponent gets to Harm you freely) to generate the maximum number of bonus dice possible.

Ultimately it's best to consider the opposed, parallel, bonus dice & Harm choices in action type as a menu of mechanical hooks to be matched against the fictional description of the action. I find the tension between the mechanics and fiction fruitful because the player is both encouraged to have some sense in his mechanical choices, but he's also encouraged to pick the action types that match what he wants to see his character do in the fiction. The Story Guide can encourage sense in this by questioning dubious choices: "Is your action really for Harm here? Seems to me that you can do that all day long without the enemy being worse for wear."

It's also good to remember in this context that the extended conflict is not very strongly tactical; the most important choices are what you want to see your character doing, and how much he is willing to sacrifice for victory. I find that the best players (in that they enjoy the game most, and the others they play with enjoy it as well) are the ones who are cooperative and agree to any combination of action types that makes sense in the fiction; that is, they value a robust fiction over trying to maneuver their own character into victory. The latter is also done as part of the player's tasks, but he won't insist on an opposed or parallel action just because it'd be mechanically favourable to him if he also can't narrate the fiction in the spirit of the chosen action types.

What all this means in practice is that the example you note about a parallel bonus dice action turning into an opposed action for Harm, it's very opposite, is pretty rare, because fiction isn't usually handled that way. Let's think of an example... Let's say that we have an argument over something important between a magical bard and a berserk warrior:
Parallel for bonus dice: I want my character to stop arguing with you and walk over to the big oak where I've stashed my lute. You'll get to continue preparing your plan, but soon everybody on the clearing can hear my soft song: I'm going to make a parallel action for bonus dice with my Music (I) Ability, singing a song that reminds the tribe about their inherent morals - this'll be useful for me later when I once more ask you to recant.
Opposed for Harm: Nuh-uh, I won't let you go when I'm winning; I'll grab you and demand an oath of fealty so that it's clear to everybody that the argument is over and you accede to my demands. It'll be opposed for Harm with my Leadership (I)!
Story Guide: One of you wants a parallel action and the other wants an opposed action. The warrior guy's action seems pretty fair to me, difficult to ignore; would you accept it, magical bard guy?
Parallel: Not as opposed - I'm just going to take the hit, he can get Harm for parallel. I'll be cool and take the shaming, he can call me a coward. I'll have time (and Harm track aplenty) to come back once I've done my song.
SG: Berserk warrior guy, will you take a parallel action for Harm? The magical bard guy seems unwilling to engage your taunts. You can't force him to oppose you, but you can change your own action to directly engage him in defence against his song - provided that it makes sense in the story, that is.

(That example would probably be easier to follow if it were a physical conflict like a battle instead of social. Well, I hope you catch the point.)

Would it make sense for the berserk warrior guy to defend against a seemingly harmless song intended to influence on-lookers and thus gain bonus dice for the next round of action? How would he do that in the story? My own call as the berserk warrior guy would be that the proposed course of action is fair - I get to shame the magical bard guy by showing him up as a coward with a parallel action for Harm, he gets a cool music scene, and then once the sun falls and stars come out we'll resume our argument. The mechanical situation might be such that this'd be disadvantageous for me, but that would be immensely rare, frankly; perhaps if the berserk warrior guy were on his last leg in the conflict and the magical bard guy were unscatched, but the berserk warrior guy actually had the better Ability ratings, then perhaps the berserk warrior guy might insist on an opposed action in the hopes of annulling whatever the opponent is trying to do with his superior result. Sounds a bit far-fetched, doesn't it?

Thanks, that clarifies things a little bit.

I am not quite sure what your example is meant to illustrate. More specifically, how can the bard do a parallel music action if the Barbarian is grabbing him and beating him up?

The example's supposed to illustrate the difficulty of conceiving of a practical situation where a defensive action would be set against a parallel action for bonus dice. The latter is by nature a round-about thing, something you wouldn't defend against directly. Taking a beating (not that the warrior in the example is beating the bard; he's confronting him and causing social Harm, but not violently) is quite acceptable if your intent is to take a parallel action - you ignore the beating (or rather, take it and then do your own thing afterwards, to be more exact) and do something else, whatever seems appropriate within the scope of the conflict.

The important bit is that the action types are fundamentally negotiated by the players based on the fictional situation, and ideally the outcome is one that strengthens everybody's visualization of the action in progress. For this reason everybody should opt for action types that make sense and match the fiction first and foremost. If you want some specific type of action, make the events in the fiction match that.