I'd like to insist on the relationship between grammar competence and 
the production of a relatively correct text by a begginer student of 
English as a second language.
First of all I've found that there is a big amount of grammar rules to 
master in order to produce a simple text. However a student doesn't 
need to use them all. It is possible to write a 100 word text, giving 
information about yourself and only use a part of the grammar you have 
learned. So one can conclude that the use of English and the knowledge 
of grammar don´t go neccessarily together.
That is frustating for a teacher to find out, because you can have 
your students write a text for you to assess whether they have studied 
and successfully learned the set of grammar rules you have taught, 
and see how they can write a good text without the grammar you were 
waiting for.
Consequently I try to ask for compositions that will make them use 
their grammar. For example if I ask for a composition on the topic "My 
last weekend" you can be sure the students have to use the past simple 
tense.However it is difficult to find a composition topic that will 
make them use the Present Perfect tense. On the other hand there are 
times when a students uses a tense that he or she wasn't supposed to. 
However his/her text asked for it.
It is amazing how live a language is! Probably too alive for a teacher 
to fully control it.