As language methods often completely ignore the language of instruction, they tend to have an irritating habit of describing absolutely everything a language does. Learners are often forced to spend too much time trying to make heads or tails of grammatical descriptions provided to them for a new language, without ever realising that the target language functions in the same way as the base language, or indeed realising it when it’s too late and the knowledge can do little to spare the learner any effort. What’s worse, is that superfluous descriptions run the risk of becoming mental debris that the learner is unsure how to apply. They may lurk in the shadows and raise their ugly heads later on, interfering with other thought processes (often in fascinating ways, mind!).
In short, we don’t need to describe everything the target language does, and what should remain unsaid will have much to do with the structure of the language of instruction. To describe certain things in the target language which are indeed the same in the base language (without a particular reason for doing so) would serve to make our learner feel less in control than they would have with less information. We will avoid burdening our learners with irrelevant observations they’re not sure what to do with, and in this way we also cue that what we do tell our learners is important.
[We often already know, without realizing that we know.]
When it comes to importing vocabulary, we are almost always using the vocabulary for something other than learning a word. When Complete Spanish opens with -al words, it is not so we can learn legal, normal and metal, but so that we can begin dissecting what vowels sound and look like in Spanish, whilst raising the learner’s consciousness of word stress, too. In the same course, when we access verbs through the pattern ‘cancelation - cancelar, we do so to highlight the infinitive and its function, to then begin establishing the infinitive as a launchpad for building other tenses. The vocabulary conversion itself is secondary to these goals. Our learner will be taken aback by all the free words, of course, but our own focus as writers is elsewhere!
Improve.