What is the "Keren" to which the Pasuk refers?
Rashi: The Keren is the principal, to which the sinner must add a fifth.
Why does the Torah write "va'Chamishosav" in the plural?
Rashi: This teaches us that if the sinner falsely claims that he has already paid the fifth and swears to that effect, he must pay the fifth plus a fifth. 1
And so on, until the fifth on which he denies is worth less than a P'rutah (Rashi). This in effect means, that he is paying a number of fifths on the initial principle.
What are the implications of "la'Asher Hu lo?
Rashi: It means to the owner of the money (article). 1
What are the implications of "be'Yom Ashmaso"?
Rashbam and Targum Yonasan: It implies that the sinner is only Chayav to pay an extra fifth and to bring a Korban Asham Gezeilos if he admits to having sinned. 1