Somebody could help me with it? I dont know English well, so...
How it'll be on Papyrus? -

scn ExampleScript

Short DoOnce

Begin OnTriggerEnter Player

Set DoOnce to 0
If Player.GetItemCount ExampleKey
If GetStage ExampleQuest 10
SetStage ExampleQuest 20
Set DoOnce to 1
EndIf
EndIf
End

Here is hopefully an examle that covers what you're looking for. I'm not sure what your DoOnce variable is for... My thought is you don't need it and instead use the Stage instead... but, if you need it, this example shows it as a property so you can pass in the value. (If it's suppose to be a Global value, then you'll need to change it accordingly.)

Define the properties in the script you are extending:

Quest Property ExampleQuest AutoMiscObject Property ExampleKey AutoInt Property DoOnce Auto

Then call this function in the event you want to respond to:

Function DoThis()  If Game.GetPlayer().GetItemCount(ExampleKey) && ExampleQuest.GetStage()== 10	ExampleQuest.SetStage(20)	DoOnce=1  EndIfEndFunction

Disclaimer: I didn't actually get to test this code... so ask if there are any errors you don't understand.

Oh, thank you. i'll try it.

You can do without the DoOnce given the quest stage is checked/changed.

ScriptName ExampleScript extends ObjectReferenceActor Property PlayerREF AutoKey Property ExampleKey AutoQuest Property ExampleQuest AutoEvent OnTriggerEnter(ObjectReference akActionRef)	If akActionRef != PlayerREF	ElseIf ExampleQuest.GetStage() != 10	ElseIf PlayerREF.GetItemCount(ExampleKey)		ExampleQuest.SetStage(20)	EndIfEndEvent