Grammar vs Vocab: where to place your focus
Since there's only so much you can teach a student at once, where should you place your focus? Broadly speaking, is it more important for a student to understand grammar or to know vocabulary (both meaning and pronunciation)?
Obviously, both are important, and students should be adept at both by the time they get to more advanced levels. But at lower levels, when teachers must limit what they teach, do more limitations fall on grammar or on vocabulary?
You can't have one without the other
Grammar isn't much good without vocab, and vice versa. Think of using language like building a house. Grammar is represented by blueprints, and vocabulary is represented by building materials. If you have grammar but no vocab, than you have a nice idea of what you want to do or say - a nice picture of a house - but you can't apply it and have nothing to show for it; there's no substance. On the other hand, if you have vocab but no grammar, that amounts to a pile of bricks; it's something substantial, but it serves no purpose.
But again, the issue becomes which do you need first? When first learning how to build, will blueprints give you the best starting point? Or would you not be able to comprehend the blueprints until you've handled some of the materials?
Vocabulary gives you something to work with
I love grammar and would like to place more emphasis on that. After all, the focus of Insights to English is on grammar. But you need something to talk about before you start, you know, talking.
If we continue the previous metaphor, we'd give some blocks to toddlers before we give them anything resembling blueprints. They might not know a whole lot of things to do with those bricks, but they can figure out some basics like putting them next to each other or even on top of each other. By the time they get their own Lego set, they'll be familiar with both the buildings (because they've seen buildings in real life) and with the building bricks that make up those buildings (because they've played enough with the blocks we gave them earlier).
Another point is that vocabulary establishes context. Without context, meaning is undefined. If you're visiting an English-speaking country and say, "Where is the ... uh ...", that's not going to help you very much. But if you show an inquisitive expression and say "train station?" you should get a helpful response. The latter has less grammar than the first, but people at least know what you're talking about, and they might be able to figure out the rest from there when your needs are that simple.
For beginners, I'd recommend starting with more focus on vocabulary than on grammar. Give your students something to work with. But as they move onward, vocabulary should give way to grammar.
Grammar clarifies meaning
Once context is established, meaning needs to be communicated. A weak grasp of grammar can lead to confusion and misunderstandings. While some simple meanings can be inferred (like with "Where is the train station?"), others are ambiguous.
Again, support you are in an English speaking country. This time, you point to a chair and say "seat?" to someone sitting in an adjacent chair. Your general meaning in understood, but what response are you looking for? Did you mean "Is this seat taken?", in which case the answer you want is "no"? Or did you mean "Can I take this seat?", in which case you're hoping for a 'yes'? There's a chance that you'll hear an answer that benefits you but don't understand it and therefore go looking for another seat elsewhere.
Or what if you're at a party and someone comes up to you and asks a question. You understood that the key words in the sentence were 'music' and 'loud'. How do you respond? Is the question "Is the music too loud?" or "Is it alright if we turn up the music a little louder?" Again, the general meanings of these two questions is almost the same, but the responses they require are opposites. If only you knew how adverbs of degrees (like 'too') work, or what comparative adjectives were, or stative vs dynamic verbs.
Once students have enough vocabulary to play around with, it's important to dive into grammar so that they better know what to do with all that vocab.
Grammar is a mark of proficiency
As students move into a lower-intermediate level (A1-B2), a focus on grammar sets students on a better path for improvement. To switch to a different metaphor, suppose that grammar structures are general recipes while vocabulary is an array of flavors. It's more impressive for someone to prepare thirty different types of baked goods, even if they are all cherry flavored, than for someone to prepare the same muffin with thirty varieties of flavors.
Or think of it this way: when you meet someone who understands all the words you say but makes a lot of grammatical errors, then meet someone who has excellent grammar but has to ask for definitions of various words, which of the two would you consider more fluent?
Even supposing there are few mistakes, a speaker (native or otherwise) who uses complex and varied sentence structures would likely be perceived as more educated than a speaker who only uses a couple simple structures, even if the latter utilizes more vocabulary. Such a perception may be faulty, but wouldn't teachers and students aim for giving the stronger of the two impressions?
Vocabulary's effectiveness wanes
There are certainly advantages to demonstrating a wide vocabulary, but the marginal usefulness of another set of words diminishes as personal vocab grows. At some point, it becomes increasingly rare that you would use any particular new word in conversation. I once used a coursebook that taught mining vocabulary at a lower level. Not that it's bad to know those words, but how often is an elementary student going to talk about mining with another English speaker (or at all), unless they happen to live in a mining town?
Or think of all the synonyms that English has. Synonyms may have different connotations, be appropriate for different settings or moods, or have other small differences, but in most cases any one synonym in place of another will be sufficient. Unless students are writing a poem or otherwise need to explore the beauty of the English language or to express themselves elegantly, the benefit of knowing twenty synonyms of one another is worth less than the benefit of knowing three alternate ways of structuring a sentence.
Of course, I strongly encourage more advanced learners to learn lots of synonyms, but at intermediate levels, it's more important to be able to communicate lots of things well enough, not to communicate a few things expertly.
Grammar comes less naturally
Perhaps a better standard is the how likely and how well learners can improve given their current skill levels. The muffin-maker can learn new recipes, but each one will take more time to not only learn, but master as well. Contrast with the baker who uses cherries with everything; she can quickly learn to substitute other flavors in her various baked goods.
As learners become more comfortable with their language skills, they're more likely to stick with patterns they are comfortable with. They're less likely to change something they've been doing for a while. I know a number of upper-intermediate and advanced students who frequently make mistakes with very common structures, but since they've been doing those same mistakes since they started learning English, they don't recognize them as wrong or see the need to change. It's hard to fix this problem, so the better option is to make sure students learn correct grammar in the first place.
On the other side, it's quite easy to learn new vocab words once you have a feel for the language. Especially if you're surrounded by English (whether you live in an English-speaking country or you regularly encounter it through media). Sure, you won't always remember the words, but you can often grasp enough of the meaning for whatever situation you're in. And if you need to, you can open a dictionary or use some translation app. There you have it: more vocabulary at your fingertips. But there's not a good way to quickly look up grammar to see which structure might be best in your situation, or what exactly someone else meant when they said a sentence in a way you're not quite familiar with.
It's better to teach grammar in school because it's harder for students to learn grammar on their own. Vocabulary, on the other hand, can readily be learned outside the classroom perhaps almost as well as inside the classroom (depending on how well and often the learners are exposed to English).
Grammar provides structure and opportunities
Both recipes and blueprints provide structure. Once you know those, you can flesh out the rest of your meal, house, or sentence with any number of things. Plug in whatever works for that situation; customize language to meet or craft your own style. But you can't start manipulating syntax or even semantics until you first have a structure to work within.
Once you've got that structure, though, the opportunities are endless.
In conclusion, a heavy focus on vocabulary is essential at lower levels and can be very effective at advanced levels. But at intermediate levels, a strong focus on grammar can be better for students' general communicative abilities in the short term and can better position students to advance toward proficiency in the long run.