Please, do not try to find direct connections between grammatical structures of English and Russian languages. You have to try to understand the logic of language. The grammar always plays together with words/context. So, «Она писала письмо.» can be translated “She was writing a letter. She had been writing a letter. She wrote a letter.” Therefore, «Она написала письмо.» would be rather translated “She finished a letter. She has finished a letter.” Just read and speak more Russian.

It depends on the context. There is no universal rule or direct match to English tenses. Examples:

"Когда я вошел в комнату, она писала письмо" - She was writing a letter when I entered the room. But "Она писала ему письма каждый день" - She wrote him every day. "Она написала письмо" - she has written a letter, but "Вчера она написала письмо" - Yesterday she wrote a letter.

K P, Russian is so tricky that you can never be certain:

Я иду в университет (right now).

После завтрака я обычно иду в университет (not now - usually).

The verbs of movement of the imperfect aspect are divided into two groups: one-directional and differently directed (they are called in Russian однонаправленные и разнонаправленные глаголы):

бежать - бегать, идти - ходить, ехать - ездить, плыть - плавать, лететь - летать и т.д.

I know it because when I was in Germany, German teachers of Russian used to make the typical mistake, they said "Простите, я тороплюсь, мне надо бегать" instead of "бежать"  and they couldn't understand the difference.

In Russian, verbs are clearly differentiated into two camps:  those which connote processes, and those which connote completion of an action.  So you are correct that "Oна писала письмо" can very often be translated as "She was writing a letter."  

However, verbs in English are flexible regarding their aspect.  The past tense "wrote" can mean either "wrote (and completed)" or "wrote for a period of time."  For example:  A.  "What did you do all day?"  B.  "I watched movies".  B's answer doesn't necessarily mean that he watched movies from the beginning to the closing credits. There is no emphasis on "completion" or "finishing." B's answer just means that he spent time watching movies.  This conclusion that we are dealing with events unfolding over time follows from the context of the question, "What did you do all day?"  

Imperfective means:
- continuous tenses.  was/is/be .....ing.
- repeated, usual etc. actions/states. Those which combine well with 'never', 'often', 'every friday', 'always'  etc. (the action you 'never' perform isn't exaclty 'repeated' action:) But within grammar it behaves the same way)

So она писала письмо is 'she was writing a letter [at the moment]'
она писала письма is most probably 'she wrote letters [often/that year]'. Depending on the contexts it may be 'she was writing letters' (or the letters).


Verbs of movement have a funny trait: they have 2 stems, one for 'continuous' and one for 'repeated' meanings. So:
Я иду в университет (right now).
По утрам я хожу в унивеситет.

Я несу сумки (now).
Я ношу мамины сумки (a boy, he means that he heps mother to carry bags from the nearby story)
Я ношу пиджак (I wear jacket).
везу-вожу, плыву-плаваю etc.etc.etc.

Now we can define perfective as 'everything else', by exclusion. It isn't much helpful though)