Lela said:
Hong: Part of what I meant with the Nymph was how would the spells come in? Would that mean that the Nymph in question would cast as an 11th Druid?
And no, I don't think is has sunk in. It doesn't seem to fit together quite right in my head.
Not a problem.
You are confusing HD and level advancement, CRs, and ECLs. These are related but quite distinct things.
A 4th level druid nymph has 7 HD (3d6 from nymph, 4d8 from class levels). This is just like multiclassing for ordinary characters, with the monster HD being treated as a "class". The abilities from the base monster and the class generally don't stack, just like with ordinary multiclassing. For example, a 3rd level sorc/4th level druid has a caster level of 3 for sorc spells, and 4 for druid spells; not 7 for everything. There are exceptions (BAB, saves and skills), but you know how multiclassing works in general, I'm sure.
The caster level for the nymph's druid spells, if we apply this rule strictly, would be 4th. However, things are messy here, and there is a precedent for stacking monster and class levels for the purpose of determining caster level (can't remember exactly where I read this). If we follow this precedent, the nymph's caster level for its druid spells would be 11th (7 for the nymph ability + 4 additional druid levels).
For the purposes of awarding XP, the nymph is treated as CR 10. The nymph is about as challenging to defeat as another monster of that CR (in theory). This has nothing to do with computing its abilities as such. CRs are rather iffy when it comes to class levels anyway, so there wouldn't be a problem in ratcheting it up or down depending on your own judgement.
ECL only comes into play if one of your players wants to play a nymph. Each monster has an ECL modifier, which is at least equal to its HD, and depending on its abilities, possibly a lot more. You add the ECL modifer to the actual class levels to determine how much XP to award. The bigger the ECL modifier, the higher level the character is determined to be, and so the less XP it gets. This is a way of keeping really powerful races in check, so that the other players in the group don't feel they're robbed of the limelight.
One of the Dragon issues had a list of ECL mods for every race in the MM. Some were okay, others were not so okay.