Prepositions are frequently prefixed to verbs. The meaning of the verb is modified by the preposition in a way that is often easily understood from the common meaning of the preposition. Sometimes, however, the matter is not so simple; sometimes the meaning of the compound verb cannot easily be determined from the separate meanings of its two component parts.
Example: ἐκ means out of, and πορεύομαι means I go. Hence ἐκπορεύομαι means I go out. But the meaning of ἀποκρίνομαι, I answer, is not easily derived from the meanings of its component parts.