In Greek, a compound verb is a verb that consists of a merger between a preposition and a verb; e.g. εκβαλλω is a compound of the preposition εκ and the verb βαλλω.
Compound verbs form their tense stems the same way as the simple verb. For example, the future of is . Likewise, the future of is .
When adding an augment to a compound verb, you will place the augment after the preposition and before the stem of the verb. Hence, the imperfect of καταβαινω is κατεβαινον.
Note: if the preposition ends with a vowel as κατα, that vowel will elide as it did in κατεβαινον.
Reduplication also occurs to the verb stem, not the preposition; e.g. εκβαλλω (present) εκβεβληκα (perfect).
Don’t confuse a compound verb with a compound predicate.