Come into effect at the start of your NEXT turn and you make the decision about what you summon and where then but you still have to select Summon I, II etc at the start:
Summon Monster I: Lidda is fighting a hobgoblin, and
Mialee casts summon monster I to conjure a celestial dog.
She can have the dog materialize in any location that she
can see within 25 feet. She chooses to have it materialize
on the opposite side of the hobgoblin from Lidda. One
round later, when Mialee is finished casting the spell, the
dog appears. It attacks immediately and gets a +2 bonus
on its attack roll because it is flanking the hobgoblin. On
Lidda’s next turn, she makes a sneak attack against the
hobgoblin and kills it. The dog disappears at the start of
Mialee’s next turn because summon monster I lasts only 1
round for a 1st-level caster.
If while trying to cast a spell you take damage, you must make a Concentration check (DC 10 + points of damage taken + the level of the spell you’re casting). If you fail the check, you lose the spell without effect. The interrupting event strikes during spellcasting if it comes between when you start and when you complete a spell (for a spell with a casting time of 1 full round or more) or if it comes in response to your casting the spell (such as an attack of opportunity provoked by the spell or a contingent attack, such as a readied action).
If the spell caster dies (not just unconscious), all Conjuration (Summoning) he had cast, end.
All other spells last until their duration expires. If the spell has indefinite duration, it lasts until dispelled. This explains how ancient gateways still operate.
*Cantrips and Orisons (Level 0 spells) are cast as a swift action and do not provoke an attack of opportunity if you can cast level 2 spells. You still have to make a concentration check if you take damage.
*Full bonus spells are given straight from the start to all classes:
Fury is a Wizard and has Intelligence of 18, he will get 4 bonus spells as a level 1 character because the bonus higher level spells simply get degraded to his highest level of casting.
*Adding a spell to a spell list (Druids, Clerics, Rangers and Paladins) requires a Spellcraft check of DC 15 + Spell level to add it to their spell list. Duration: 1 hour per spell level. This can only be attempted once per level.
*Teaching a spell to another character takes a whole day per level (half day for cantrips and orisons) and the Spellcraft DC is 5 + Spell level. This can only be attempted once per level.
*Casting spells from scrolls of higher levels increases the Spellcraft DC by 5 per level.
*Custom Miscast Chart (a.k.a. “Fun Chart”) is used if the concentration is failed by 10 or more.
*If using any perform skill, you must physically perform (sing, dance, make up a poem, etc…)
*Starting at level 4, Assassins, Duskblades, Paladins and Rangers get cantrip or orison spells. The spell progression is the same as a Druid or a Cleric of 3 levels lower than the character level.
Eximere a Level 4 Ranger has 3 orison spells and 1 level 1 spell due to his +1 Wisdom.
Improving a spell (Without feats):
You may also research/develop your spells to slightly improve them. A good example of this is Fury’s Mage Armour. This spell functions exactly like Mage Armour except for the following:
It provides +2 Enhancement bonus to hide checks while in sandy areas. He was able to improve this spell as he has spell mastery of it, has moderate use of it and spent time researching.
Improving a spell can be done by any spell caster (although it comes easier to those with spontaneous casting abilities such as Bards, Sorcerers and Favoured Souls.) Improving a spell does not always guaranty that the benefit will actually be of any significant use.
Example: Clerics of Kord, when casting Bull’s strength on their allies also give their target ally a +2 Enhancement bonus to Fortitude save vs poison for the duration of the spell against the first poisonous attack. Unfortunately this bonus only works against the weaker range of poison.