Second Aorist Active Indicative
There is no difference between the functions of the first and second aorist. They are simply two different ways of forming the same tense. Some verbs use one way, others use the other.
This kind of difference is typical of many languages. In English most verbs have past tense forms that end with –ed such as closed, helped, and challenged. Many verbs, however, do not form their past tense in this way. The past tense form of teach is not teached, but taught. The past of go is not goed but went.
The personal endings for the Greek second aorist are attached to the second aorist stem, and that stem can vary significantly from the one used in the present tense of the same verb.
Second Aorist Stem
The second aorist stem is usually, but not always, shorter than the present stem. Compare λαμβάνω, for example, to its aorist form, ἔλαβον. The aorist stem is –λαβ- (discovered by removing the augment and the ending), whereas the present stem is λαμβαν-. The aorist ερον and present εὑρίσκω reveal a similar relationship. The second aorist stem of ἔβαλον is -βαλ-, whereas the present stem is βαλλ-. The second aorist stem of ἔγνων is –γνο-, whereas the present stem is γινωσκ-.
Another fairly common difference between a second aorist stem and the same verb’s present stem is that the present stem has a long stem vowel while the aorist does not. For example, the aorist root of ἔλιπον (-λιπ-) is lengthened to λειπ- in the present tense (λείπω).
Whenever you encounter a second aorist stem in the vocabulary list, you must always compare it to the present stem (lexical form). You will need to recognize both stems in order to read Greek effectively.
John 17:25
ἔγνω 3rd Aorist Indicative Active Singular
ἔγνων 1st Aorist Indicative Active Singular
ἔγνωσαν 3rd Aorist Indicative Active Plural