It is clear that second language acquisition (SLA) is a complex issue. There is no
single comprehensive account of SLA, but only a number of incomplete and
partial accounts. It would indeed be difficult for one theory to account for the
entire range of second-language phenomena, especially if one considers also
personality factors and environmental factors. The trend has been to move away
form a general theory of SLA to theories of more limited scope. This makes the
task of the applied linguist an extremely difficult one. He often has to cope with
contradictory implications. UG and cognitive theory, for example, differ on the
issue of practice. The UG model sets minimal store by practice: in principle a
parameter can be set by a single example for ever more. Cognitive models see
language as the gradual accumulation of preferred ways of doing things, making
practice an essential requirement (Cook, 1991, p.123).