From The Art of Teaching Spanish: Second Language Acquisition from Research to Praxis edited by Rafeal Salaberry and Barbara A. Lafford:
In an interesting small-scale study, Seliger (1979) investigated the relationship between grammatical rules of thumb and performance and found no connection between the two. Learners knew the correct rules but still could not apply them, while others, including natives, did not know the correct rules but still could produce the correct forms.
This makes sense to me. Explicit knowledge of a "rule" does not mean that you'll remember how to produce it, especially in spontaneous speech. It is hard to apply abstract knowledge like that, especially if the L2 input is lacking in examples. Here's an example of a "rule of thumb" I found for Spanish regarding where to put the stress accent:
- If a word ends in a vowell, 'n' or 's' the stress is usually on the next to last syllable.
- If the word ends in a consonant other than 'n' or 's' the stress is usually on the last syllable.
- If the stress on a word doesn't follow the first two rules, then the syllable that is stressed needs a written accent mark on the vowel.
That makes...enough sense I guess, but it is really abstract. Especially if students are doing something like reading outloud, it is totally useless for them to stop and say "what does this word end in? where is the stress." Whereas, someone like a native speaker with a lifetime of comprehensible input knows where the stress goes because she or he is used to hearing the word correctly. I'm sure native speakers would not know this rule. I am only passingly familiar with it and I'm a non-native speaker so, you know, it's not a popular one. And, it's really hard to remember.
So, if "rules of thumb" are stupid, what is the alternative? According to the article the first quotation came from, the answer is to link it to content. That seems like a good answer. Say students learned a few nursery rhymes or proverbs early on in their language education. Then they could have something memorized with the applied rule that they could refer back to. Proverbs also have the added bonus of making learners culturally competent.
Finally, this is, in my view, a really important concept to keep in mind when teaching language. I remember learning a lot of rules like this throughout language classes, I am pretty sure they did not make me better at the language. You know what made me better at Spanish? Reading a lot of books and watching even more television.