I am not sure I understand exactly your question.
When we use a preposition before a verb, sometimes we keep the ancient form of the verb. But it is not necessary for all the cases.
Βάλλω > βάλλω, παραβάλλω
Μένω > μένω, αναμένω
Most of the propositions are the same with those in Ancient Greek. (Παρά, κατά, υπό, εκ, μετα, επι etc) and we use them in the same way with no changes.