The Esperantists who write to me are split fairly evenly into two groups: the ones who insist that Esperanto has only sixteen rules (the set Zamenhof originally published under the heading “Complete Grammar of the International Language”), and the ones who insist that nobody believes such nonsense.
The first group seem to be labouring under the delusion that the incompleteness of that ruleset is a good thing, as if grammar was a load of unnecessary red tape imposed by some cabal of authoritarian schoolmasters. Surely (they say) there's no need for petty regulations defining how syntax or allophony work – all that's needed is a rule saying “you can do it however you like as long as it's clear what you mean”. Unfortunately, the way of expressing yourself that feels right to you is just going to be the one that follows the conventions you're accustomed to, and the only people you can reasonably expect to share that feeling are the ones who grew up speaking a very similar language (in other words, people you hardly need Esperanto to talk to). Trying to communicate complex ideas across a real language barrier in a tongue where you improvise the grammar as you go along would be about as profitable as playing cards at a table where all the participants have their own opinions about what game they're playing!
Nor is devising a grammar a matter of working up a few simple rules based on universal logical principles; that isn't what natural languages are built out of. (After all, if there was an obvious objectively best and clearest way of expressing things that people from all over the world agreed on, languages wouldn't be different in the first place.) No; grammars for fully‐functioning human languages provide an artificial, systematic, two‐way mapping between the set of possible meanings and the set of possible utterances, which is an enormous amount of work for a rulesystem to do! So if you genuinely want to minimise the total number of rules, you can't afford to let the list include drivel like “Every preposition in the international language has a definite fixed meaning”; instead it would be more likely to start with a bunch of high‐level general principles along the lines of “All head/dependent structures are strictly left‐branching”. Fortunately, learners don't need to be able to recite them as commandments as long as they develop the habit of following them.
Far from making languages hard, it's grammar that makes them speakable, and the fact that some potential utterances are forbidden as ungrammatical makes a language easier to use. While you're listening to somebody speaking, you work out what they're trying to say by unconsciously building up a mental model of what kinds of valid sentences are compatible with what you've heard so far, and that's largely a process of progressively eliminating possibilities until you're left with just one. When you hear the word the, you know it's the start of a noun phrase; if it's followed by can, you automatically disregard the interpretation that it might be the auxiliary verb meaning “be able”, because that would be ungrammatical. You may not be conscious that you're applying any such rules, any more than you're aware of the way you constantly adjust the conformation of your larynx while speaking; it's just part of your fluency in the language. Zamenhof failed to specify anything close to an adequate supply of rules to make his creation usable; any time two people succeed in communicating in Esperanto, they do it not by obeying the rules explicitly stated in his “Complete Grammar” but by falling back on the language's immeasurably larger system of tacitly assumed additional rules – a system built on the compatible usage habits of its European early‐adopter community, now converted into entrenched idioms that outsiders are expected to absorb largely by osmosis. (Babies are good at that, but adults mostly aren't.)
Take word‐order rules, for example. When Germans choose to phrase a sentence as morgen tanzen wir = (literally) “tomorrow dance we” instead of wir tanzen morgen = “we dance tomorrow”, it may feel to them as if they're expressing their creative freedom to make the sentence flow pleasantly, but what they're really doing is habitually abiding by strict grammatical constraints that outlaw most orderings and assign systematically varying connotations to the valid ones. Newcomers to Esperanto with disparate habits of this sort are encouraged to just carry on following the rules they're used to – the good news is that they can usually rely on the language's word endings to keep an utterance like morgaŭ dancos ni intelligible. What nobody mentions is that this also implies some bad news: if every order is equally normal, no order has any special stylistic effect!
That's not how things work in natural languages with “free” word order. That name, with its suggestion of grammatical anarchy, is misleading; if anything, these languages have more word order rules than usual. It's just that the syntactic order rules (like “the subject comes first”) can be overruled by other, subtler ones that deal with information structure, distinguishing the elements of the sentence that are the focus of interest from the parts that are unimportant or already known. In many languages, the rule of thumb is that important stuff moves towards the start of the sentence, but it's also possible to put the “topic” of the sentence in final position, and often the emphatic form is simply the opposite of the normal order, whatever that is. Unfortunately, Zamenhof failed to recognise that any such rules were required. Instead he left Esperanto in a state that would require us to treat all word‐order variations as canonically meaningless, because no matter what peculiar order the sentence is scrambled into, that might be the neutral default in the speaker's native tongue.
And yet there nonetheless exists Esperanto poetry that relies on the assumption that speakers will all react similarly to a particular “elegant” and “euphonious” phrasing. This is because the dogma fed to learners is a lie: Esperanto does have a consensus system of word‐order rules – and as usual, they're basically the ones that the first Esperantists had grown up taking for granted. For instance, adjective plus noun goes in that order by default, so doing anything else implies special emphasis, which can mean the difference between mi faris nur unu negravan krimon = I only committed one minor crime and mi faris nur unu krimon negravan = I only committed one minor crime. This is a rule that learners accustomed to postposing all their adjectives need to be warned about, but while advanced grammar references written in Esperanto might acknowledge it as existing, beginners are still taught that adjective placement never affects meaning.