الفعل الماضي و المضرع, و المصدر

بسم الله
ها هذا صحيح
أخطر / يخطر / إخطار

هو أخطر الزبون أمس
هو يخطر الزبون الآن
"إخطار لزبون"

أَحْضَرَ | يُحْضِرُ | إِحْضَارُ 

we name that verb: الفعل المزِيد because we add to the real verb of it, an Alif to have another sense. 

أحضر = أ + حَضَرَ 

حَضَر  يحضر حُضُورٌ means attended

the second you wrote means : brought, got. Logacilly to understand it. We usaully add alif to have another sense of the origine one, the mazeed verb will take another sense like that : the sense of the first one + object(thing) (in sense with something or someone)

example: 

حضر الزبون البارحةَ = the constumer attended yesterday.

أحضر الزبون البارحة نقودا = the comstumer brought yesterday some money.

So logically the constumer I didn't know what the constumer has, I only understand he's attending and that's all, but the second I describ with more details, he's attending  and with something (some money).

another example:

غَضِبَ = be angry 

أغضب = make angry 

غَضِبَ الأبُ على ابنه = the father is (gets) angry about his son.

so logically the second with alif, should be related to something or something, alright who or which? here it's the son, and the father got angry about son's doing.

أَغضب الإبن الأبَ = the son makes angry the father.

In otherwise, alyways use examples to understand how you can use the words in Arabic. :) 

الفعل الماضي و المضرع, و المصدر

بسم الله
ها هذا صحيح
أخبر / يخبر / إخبار

هو أخبر الزبون أمس
هو يخبر الزبون الآن
"إخبار لزبون"

أظن أن الفعل المقصود هو أخبر والله أعلم