Wizards must choose to apply the metamagic feat to their spells while preparing the spell? If yes, can they just cast spells with the metamagic feat this way?
Sorcerers and Bards
When sorcerers and bards cast a spell using a metamagic feat, they use a full-round action. That is ok.
Will they use then the previous and the new rank of the spell from their daily maximum? If they use more than one feat to a spell it take a full-round for every feat applied?
Clerics and Druids
What about clerics and druids using metamagic feats in their spells, they must prepare the modified spell before or can choose while casting? When they cast a modified spell, they take a full-round action?
In case Zachiel hasn't made it clear enough...
These Are Mainly Examples
Excludes corner cases.
It's almost all in the SRD, albeit it's in the feats section, where metamagic feats are detailen, and not in the spellcasting section where one could expect to find it.
This is the relevant part:
The reason for sorcerers and bards needing more time to cast the spell is a balance one. Casters who prepare spells must choose in advance which spell they want to apply the metamagic feats on, which makes them occupy a higher slot.
As a consequence, Clerics and Druids (who prepare their spells) don't usually need the extra round, except if they apply the metamagic feats to the spells they cast spontaneously, such as the converted healing or summon nature's ally spells