From such evidence, both Pienemann and Rod Ellis propose what they refer to as a Teachability Hypothesis, ie teaching will only have an effect if the learner is developmentally ready to acquire the grammatical structure in question. Pienemann has more recently developed his ideas into what he calls a Processability Model. He argues that we process different linguistic elements with varying ease. For example, he claims that typically, learners' attention will be drawn in the first instance to elements which occur at the beginnings and ends of language units (eg a clause or sentence). This might explain why learners of German first discover that adverbials can be placed at the beginning of a clause and then that non-finite verbs must move to the end. It may also explain why learners tend to prefer external negation (see section 2.1.4.1). Movement of words into positions within the sentence is less 'perceptually salient' (less easy to notice, less easy to process) so this might explain why verb inversion (the Verb-second rule in German) is acquired after verb separation (the movement of non-finite verbs to the end) (see section 2.1.4.2). Thus, learners process salient features first, and only then are able to process less salient ones.

Increasingly, the role of cognitive factors - the way the mind processes information, the way we allocate our limited attention - are being investigated in SLA. For example, rather in the same vein as Pienemann, the American researcher Richard Schmidt has claimed that what we are able to notice in the L2 input determines what we are then able to 'pick up' implicitly (see section 2.3.3.4). What is quite clear to all teachers and learners of second languages is that some elements of the target language are more difficult to pick up than others. The problem is understanding - theoretically - why this should be.

Learners themselves often find short-cuts around grammatical complexity by learning frequently heard phrases as 'chunks', ie as if they were long words to be accessed directly from memory, rather than involving grammatical computation.