1. My Magic person has spells in his spell book, but cannot cast any:
Spells must first be 'memorized' before they can be cast. Learned spells are listed on the right hand page in the spell book. The left hand page shows spells memorized, or has empty outlines the size of the spell icons. L-clicking on a spell on the right hand page causes it to fill one of the to-be-memorized locations on the left page (see BG manual, p27), assuming there is an empty slot on the left hand page. You must rest successfully for 8 hours before spells will actually be memorized.
If the spell book's left hand page is completely blank (i.e., there are no to-be-memorized [empty outline] slots), then your character has not attained a high enough "experience" level to cast at this "spell" level. The following table shows the maximum spell level allowed to be memorized based on a character's experience level.
Click on your spell caster. He/she possesses a moonlight shape near the middle of the menu at the bottom of the window. Click on it and it will show which spells are available.
The actual number of spells allowed to be memorized for a given spell level can be found in the Spell Progression tables (5a-c) on page 140 of the BG manual. Note that the number of spells/level (table 5a-c) may be modified by either the character's Intelligence (Wizards) or Wisdom (Priests) as shown in the table on page 130 of the BG manual.
Bards MUST REMOVE ARMOR in order to cast spells
2. How Does a Mage Learn New Spells
When a Mage reads a scroll (click on the scroll in his inventory), the "Write Magic" option will be offered if the Mage has room in his spell book and does not already have that spell written into the book. Clicking on "Write Magic" will attempt to learn (write to spellbook) the spell based on the Mage's "% to Learn" ('Mild Cheat' - save prior to attempting to learn a spell as it may fail).
The maximum spells/level that a Mage can write into his spell book is determined by his Intelligence (see "Max Spells/Level" column, table 1 on page 136 of the game manual) as is the highest level of spells a mage can learn ("Spell Level" column in same table).
Scrolls can be found in various places, recovered off the bodies of vanquished enemies, purchased in stores/shops, received as quest rewards, and 'lifted' from the pockets of unsuspecting npc's (if your thief is good enough).
3. How Does my Priest Learn New Spells
Priests automatically learn new spells when they level up in accordance with the Spell Progression table 5b, page 140, of the game manual. But not all Priests will learn the same spells. Priests learn spells based on the God they worship. For instance, Branwen (Cleric) and Jaheira (Druid) will not learn all the same spells, but there will be some overlap.
Priests get to learn additional spells/level based on their Wisdom (see "Bonus Spells" column, table 1, page 136 in game manual).
In Baldur's Gate, Priests will not be able to learn any spells beyond spell level 4 due to the game's experience cap.
4. Can my Paladin (or Ranger) Learn Spells
No. Level 1 Priest spells are learned by Paladins at experience level-9 and by Rangers at experience level-8. Due to the 89,000 experience point limit in Baldur's Gate, Paladins and Rangers can only attain experience level-7. With the TOSC add on, Paladins and Rangers can attain experience level-8, so 'maybe' (don't know, haven't palyed TOSC) Rangers can get level-1 Priest spells.
5. Favorite Spells