"I am writing a new letter."
Translation:J'écris une nouvelle lettre.
pls read this about rules and exceptions: http://french.about.com/od/grammar/a/adjectives_4.htm
The point is that "je suis écris" = "I am write", so you see that it does not work.
This is the rule: In English, to mean that an action is in progress at the time you speak, you use the continuous verbal form, ie verb BE + action verb in the gerund form (-ing). o he is eating means he currently eats
In French, this verbal form does not exist (directly translated “il est mangeant” is incorrect).
Therefore, you can translate either “il mange” or “il est en train de manger”, where the construction verb être + en train de + infinitive correctly expresses the English continuous form.