I am not sure, but I think for nouns in general (e.g. ποίημα) the accent must be memorized as there is no way to predict what syllable it will fall on. However, for verbs, it is predictable, for example, past aorist verbs are usually proparoxytone- the accent is usually on the antepenult (third to last) syllable (e.g. χρησιμοποίησα). In contrast, past participles are usually paroxytone- the accent is usually on the penult (second to last) syllable (e.g. χρησιμοποιήσει). These are just my observations as a learner- let's see if a native speaker can confirm...
In general, the accent falls as far back as possible (away from the end of the word, closer to the beginning) -- this means on the antepenultimate if the final vowel is short, the penultimate if the final vowel is long.
But this general scheme is muddied up by various phenomena such as the fact that modern Greek doesn't have distinctive vowel length in pronunciation; vowels being contracted together into one syllable; and analogy in modern Greek producing words that are accented on the antepenult but written with a letter that would be long in ancient Greek (e.g. η δύναμη).
And even in Ancient Greek, you can't tell whether an α ι υ is long or short just by looking at it -- for example, in ποίημα it's short (as with most(?) other neuters), thus the accent is on the antepenult, but in κυρία it's long (as with many feminine nouns), thus the accent is closer to the end than in κύριος, and in γλώσσα it's short again (despite being feminine).
'να γράφω' = to be writing.
It doesn't make much sense in this sentence but could be used in another, such as: When you come home from work, I am going to be writing = Όταν γυρίσεις σπίτι από τη δουλειά, πρόκειται να γράφω. (Although, no one ever would say 'πρόκειται να γράφω' in this case, you'd simply say 'θα γράφω'.)
So, I had the same question as I.Schmidt1, and I think I understand, but just to confirm πρόκειται να γράψω means "I am going to write (something and be finished with it) while πρόκειται να γράφω means "I am going to be writing (something as a process but not necessarily finish). Yes?