ASPECT:   How do we view the action?
Are we concerned with 'simple fact' or repetition, progress, result?
Aspect Action Time
in progress   I was daydreaming    past 
 I am writing, listening to the radio....   present 
 I shall be writing for quite some more time   future 
general,
repeated		  I used to drink wine, I often quarelled   past 
 I write books, I drink beer rather than wine, I sleep well at night   present 
 I shall continue to do so, often, repeatedly   future 
completed  I had finished when the clock struck midnight   past 
 I have spilled some coffee over my keyboard
    (it's all gooey now) 
 the action is completed in the present: present perfect		  present 
 I shall have finished by the time you get back   future 
specific,
simple fact		  I went to the theatre yesterday and saw 'Electra'   past 
 I shall leave tomorrow, full stop, no comment, no objection!   future 

How important is all this? Since there is no difference in the verb used in

'I wrote 3 letters on Sunday' and
'I often wrote 3 letters on Sundays.'
Well, there is no difference in English, but there IS a big difference in Greek.

Simple fact requires a tense called AORIST*

aorist indicative (indicating simple aspect of past events)
aorist  infinitive    
aorist  participle   
aorist  imperative 
aorist  subjunctive
aorist  optative     

Which sounds worse than it is because the aorist only means one thing:
simple, specific action (or state)



I wrote my sister a letter yesterday. (specific letter, aorist indicative) versus:
I often wrote letters to my sisters. (repeated action, so non-specific: imperfect)

I must send this letter off now. (send specific letter, aorist infinitive)
I must send letters to her regularly. (present infinitive for general vagueness)

Having learnt (1) his lessons, he whistled (2) his dog and went (2) for a walk.
      (1)simple fact, specific chore: aorist participle and
      (2) simple actions snapshot-like (what happened next? flash, flash): aorist indicative
While learning (3) his lessons he listened (4) to music.
      (3) learning in general: present participle
      (4) continuous or repeated listening: imperfect

* Aorist means without boundaries, considered without any thought to completion, duration, or repetition. Aorist refers to mere, unadorned, straightforward, specific fact, action or state.
I worked as a teacher, I did, aorist, simple, straightforward statement of specific occupation/state.
I lived in Switzerland. Aorist, simple statement of fact.
I got fed up with the climate and moved to Portugal. Aorist: simple fact: I got fed up. So what happened next? simple fact: I moved!
I used to live (1) in Switzerland, but now I'm living (2) in Portugal. (1) Imperfect because I'm not living (2) there any longer, the focus has changed: no longer simple statement of facts but 2 ongoing states, my past living in Switzerland and my present living in Portugal.