Prefixes change the meaning of a verb. The change in grammar goes along the change in meaning.
All separable prefixes have a matching preposition and the corresponding verbs work very similar to English's phrasal verbs. You can guess their meaning from the "included" preposition. The purpose of these verbs is often enough creating a verb phrase with two prepositions.
Er stellte es vor eins ab.
Er hat es vor eins abgestellt.
He shut it down before one o'clock.
Stellen means to put, abstellen means to put down/to shut down. This is combined with the preposition vor (in front/before).
Er stellte es ab eins vor.
Er hat es ab eins vorgestellt..
He put it in front from one o'clock on.
Vorstellen means to put in front. This is combined with the preposition ab (down/from).
Please note the order of phrasal and "real" preposition in the English sentence is the opposite of the order of separable prefix and preposition in the German sentence. This leads to much confusion for English speakers learning German (and vice versa).
The meaning of verbs with non-separable prefixes isn't easy to guess, and the prefix be- is the most complicated one.
jemanden raten → to guess someone (from a group)
jemandem (etwas) raten → to advice someone (something)
jemanden beraten → to advice someone
jemanden stellen → to oppose someone
jemanden bestellen → to summon someone
jemanden fragen → to ask someone
jemanden befragen → to survey someone
Do you see a pattern? I don't.