Minor Spoiler !
Rangers start gaining priest spells only from level 9 on, so they won't in BG I/TotSC.
Your question is how to select spells from the right side of the priest's spellscroll, so they will fill the slots on the left side ? Just left-click the one you want to select on the right hand side. Right-clicking will give you the spell description, btw.
You can cannot enter Baldur's Gate before chapter five, thus before you have reached a certain point in the main plotline. Does the name "Cloakwood Forest"
sound familiar to you ? And it is a big forest, so big it won't fit on one single map. Still certain you have been everywhere ?
Tell us if you want to be spoilt more openly.
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So long !
R³ - Co-president(s) of the Club of Broken Hearts