Words are like people: some are strong and some are weak and have to lean on others for support. These weak ones are called en-clitics. They have to lean on what's before them. So together with their strong neighbour they form one unit and leave stress and written accent to their stronger partner. Let him or her speak up. It follows quite naturally that:

If there is nothing in front to lean on they have to stand up for themselves and carry a written accent to show they are stressed and strong:
   εἰμί
   διὰ τί; φησί (Why? says he) 1st word after punctuation

When followed by another enclitic they feel strong and help their weak neighbour, i.e. they are stressed and show it through the written accent they proudly bear. But only if they are long enough, have 2 syllables, otherwise they reacute weakly standing on one leg, precariously..
Example:  οὐ δυνατόν ἐστί μοι πονεῖν  (it's not possible for me to work) while, without the 2nd enclitic μοι:
οὐ δυνατόν ἐστι πονεῖν. (it isn't possible to work)

So what happens to their stressed neighbours?  

1. ἄνθρωπος  +  ἐστιν.
   The Greek found saying this in one go unaesthetic, so we get
   ἄνθρωπός ἐστιν 
   the πός ἐστιν pretending to be a separate antepenult. For the sake of pronunciation! Antepenult takes 2 acute accents, one normal, one extra on last syllable.
    
2. What about  δοῦλος +  ἐστιν ? Same as for above, for the sake of easier pronunciation we get:
   δοῦλός ἐστιν,
   the λός ἐστιν  pretending to be a separate antepenult.
   Long circumflexed penult takes two stresses, its normal long circumflex and an extra short acute.
    
3. And what happens to ultimas? Well, nothing obviously, acute last syllables stay acute:
      ἀγρός ἐστιν  
   and long circumflexed ones stay long circumflexed:
      Ζεῦς ἐστιν
   and enclitics stay what they are, weak enclitics. Easy!
    
4. And short, acute penults? Such as δένδρον?
   Nothing happens to them, but a two-syllabled enclitic will get its confidence back and announce to all and sundry: I'm here, I'm carrying a written accent to prove it:  δένδρον εἰμί  (I am a tree)

If you want to pronounce Greek, enclitics and all, then by all means do try and remember them (as they crop up in lessons). If not, forget about them for the time being and concentrate on things of more immediate interest, like reading for meaning.