Hi,
I have question about workflow and Trigger.I create a custom object ="Product" on opportunity object .I create three (3) fields on custom object = Product and the filed description is below:
Field # 1:Application Received By(which is lookup(user)field)

Field # 2: Application Received (which is date filed)
Field # 3 : Closing Stage (pick list field; option s are  received, closing)
Field # 4: Closing Stage Owner ((which is text field)

Field # 5: Process (which is date field)

Requirment # 1:

The reuirment is that

 If the “processing” field not Null (date picked by user)

 Then

Set

Condition # 1:  “closing stage = closing”

And

Condition # 2: “closing stage owner = null (blank field)”.

Requirment # 2:

The reuirment is that:
If Application Received By not null (user selects some name of the user in the field)
Then

Set

condition # 1: Application Received = todays date (date when user pick enter this record )

condition # 2 : Closing Stage = recieved
and 
condition # 3 : Closing Stage Owner = Application Received By

Example: If  Application Received By = steve then according to workflow it should update the Application Received =  todays date(10/19/12) ,Closing Stage = recieved and Closing Stage Owner =steve

I create workflow for both of the requirments  and i am able to satisfy the requirment #1 and 2 .  when i select the requirment # 2 first then working fine and if I edit the record and change it to requirment # 1 and this satify the reuirment but the problem is that if I remove the the date from Processing date filed nad levae the field blank then the Closing satge field should chagne from “Closing “to “received” but its not going   . 
I am new to salesforce and I don’t know that how to complete this requirment. Do I have to modify my workflow orcreate new workflow or do something else (trigger).If I have to create trigger then please help me for this because I don’t know that how to create trigger

thanks