There is no real difference in meaning or use between finish (verb) and finished (adj) or between complete (verb) and completed (adj). Both sound very natural in spoken English:

• Is your work finished for the day or do you still have some to do?
  
  
• Have you finished your work for the day or do you still have some to do?
  
  
• Can I read the manuscript of your latest novel? ~ No, sorry, it's not completed yet.
  
  
• Can I read the manuscript of your latest novel? ~ No, sorry, I haven't completed it yet.

Note, however, that complete as an adjective with the slightly different meaning of whole or entire is more frequently used than completed as an adjective, meaning finished:

• No house is complete without carpets on the floors and pictures on the walls.
  
  
• If you think I can handle all this work on my own, that shows a complete lack of understanding on your part.
  
  
• With only one hand on the steering wheel he was not in complete control of the car he was driving .
  

Adjectives ending in -ed

A large number of adjectives in English end in -ed. Many of them have the same form as the past participle of the verb:

• Your behaviour this evening has disappointed me.
  
  
• I am disappointed with your behaviour this evening.

They indicate that something has happened or is happening to the person referred to. Thus, a child who is spoilt is a child who has been spoilt by something.

Here are some more common adjectives which have a similar meaning to the related verb:

• It worries me that Jack stays out so late every night. I am a very worried mum.
  
  
• I would be interested to know if you are planning to visit Greece this summer.
  
  
• That interests me because I shall be there throughout August and September.
  
  
• You will embarrass your father if you dare to wear clothes like that.
  
  
• She came down the stairs wearing jeans with holes in them and I have never been so embarrassed.

Occasionally, the adjectival form has a meaning which is different from that of its related verb. Compare the following:

• I spotted her through the crowded room. She was wearing a spotted dress.
  
  
• We advanced through the jungle as quickly as we could as we needed to reach the clearing by nightfall.
  
  
• The cancer was quite advanced and he had only a few weeks to live.