Learn Farsi: وقتی قاضی احضارم کرد، من قسم خوردم. - When the judge summoned me, I swore an oath.

In Farsi compound verbs, the pronominal enclitic (like -am for me) usually attaches to the non-verbal part of the compound, rather than the light verb at the end. Here, ehzar (summons) takes the -am ending, making ehzaram kard (summoned me).

If I am the one being summoned, why does the verb end in kard instead of kardam?

Because the subject of this first clause is the judge (ghazi). The judge is the one doing the action, so the verb is kard (he/she did). You are the object receiving the action, which is shown by the direct object enclitic -am attached to ehzar.

Do I need to add ke after vaghti (when)?

You can, but it is entirely optional. In spoken and written Farsi, it is very common to drop ke and simply start the clause with vaghti, just like in this sentence.

Why is the verb khordam (I ate) used for swearing an oath?

In Farsi, the light verb khordan (to eat, to drink, or to strike) is used in many idioms. The compound verb ghasam khordan literally translates to 'eating an oath', but it is the standard and natural way to say 'to swear' or 'to take an oath'.