Hi Jessica, "Runneth" is an archaic (no longer in use) form of the verb "run". It is believed that the ending - eth (used in Middle English and Early Modern English) was the default ending for verbs conjugated in the third person present simple form. In accordance with this belief, "my cup runneth over" is simply an old way of saying "my cup runs over". I hope this was helpful. 😇

Sorry, not sure about "my cup ranest" over. Might just be "my cup ran over". Don't know. The "-est" ending goes with "thou" (second person familiar) so it might be wrong for "my cup".

The simple past was formed by adding -st or -est. So it would have been "My cup ranest over". (The -e is optional.)

Each verb has only one past participle. The past participle does not conjugate, although the helping verb does, so you would say "My cup hath run over".

Runneth is no longer used in English. Now days we would say my cup overflows.