“Our use of language depends upon two capacities: a mental lexicon of memorized words and a mental grammar of rules that underlie the sequential and hierarchical composition of lexical forms into predictably structured larger words, phrases, and sentences.”
Michael T. Ullman: The Declarative/Procedural Model of Lexicon and Grammar
Journal of Psycholinguistic Research
Volume 30, Number 1 / January, 2001
According to Mr. Ullman the lexical knowledge (the association of meaning and sound into morphemes, irregular word forms etc) is linked with the declarative, temporal-lobe system. Declarative learning deals with learning active facts that can be recalled and used with great flexibility. Declarative memory is memory for facts. It is associated with the temporal lobe.
He links grammatical knowledge to the procedural memory system:
“In contrast, the acquisition and use of grammatical rules that underlie symbol manipulation is subserved by frontal/basal-ganglia circuits previously implicated in the implicit (nonconscious) learning and expression of motor and cognitive skills and habits (e.g., from simple motor acts to skilled game playing). Morphological transformations that are (largely) unproductive (e.g., in go—went, solemn—solemnity) are hypothesized to depend upon declarative memory. These have been contrasted with morphological transformations that are fully productive (e.g., in walk—walked, happy—happiness), whose computation is posited to be solely dependent upon grammatical rules subserved by the procedural system.”
The procedural memory system specializes in non-conscious learning and control of motor and cognitive skills (habit learning). It is associated with the striatum and frontal/basal-ganglia. Procedural memory is memory for skills (riding bicycle, dancing). There is convincing scientific evidence that highly automatized language skills are processed at this level.
The old saying that learning a language is like learning to ride a bicycle seems to be supported by a scientific explanation. They also say that learning to dance is very much like learning a new language.
From the above one could extrapolate that excessive rote memorization of grammatical rules is completely unnecessary and counterproductive. On the other hand language is a lot more complicated than a bicycle (or a dance). With a bicycle one at least needs to know where are the pedals, seat and handlebars. Dance manuals exist for a reason. The parts of speech and their use are not always immediately recognizable in a foreign language. Nor is it easy to understand how they dance or interact together. A good rule of the thumb is that the more complicated a piece of equipment or a skill that needs to be learned, the thicker will be the manual (and more pressing the need to consult it). Research (Erlam 2003) comparing learners who receive deductive (rule presentation and metalinguistic information) or inductive (focus on form with no explicit grammar) instruction "shows that learners receiving deductive instruction perform better on both listening and reading comprehension and written and oral production tasks". From the "Handbook of Educational Psychology" by Patricia A. Alexander and Philip H. Winne
In addition one could also conclude from the above that extensive and intensive use of dictionaries and word lists is a good way towards language proficiency.