UG goes some way towards explaining how aspects of l2 knowledge are
acquired in the first place. But this is not what teachers are solely concerned
with. Ellis (1990, p.175) says: 'Teachers are concerned with both how knowledge
is acquired and crucially, with the learners' ability to make use of this
knowledge". In this regard, Taylor (1988, p.166) makes a worthwhile distinction
between competence and performance. He suggests that the term 'competence'
be used for 'knowledge', while 'performance' be used for 'ability to use
knowledge'.