What makes ĉu necessary?

Question

This is not a question about the actual rules but a hypothetical one: had ĉu never been introduced to the language, but all the rest was the same, what, if any, would be some ambiguities this would cause? Should there be none, has it been documented why Zamenhof did not eliminate such auxiliary word from the language (just like he did eliminate, e.g., the indefinite article)?

I used to think that a word to mark a question was necessary, but in written text that's quite literally what a question mark does and in speech we have intonation. My current guess is that there may be some compound sentences in which the understanding would differ depending on whether ĉu appears in the dependent or independent clause or something but I can't come up with any working examples.

Answer 1

It seems that there are a few different subtleties in your question, and at least a few of them are subjective or otherwise potentially off-topic. For example, if you're asking about why Zamehof did something, to me that's a different question from why this same something is a good idea.

As for Zamenhof, it seems obvious that he was familiar with several languages that included a ĉu-like particle and it seemed useful to him. As a side note, question particles are known in a rather wide range of languages, many of them unknown to Zamenhof.

As for potential ambiguities, again it seems obvious that a question understood as a statement (or vice versa) is a pretty big ambiguity. ("There is a monster under the bed" vs "Is there a monster under the bed?").

From a "why is it useful" standpoint, consider the most commonly proposed alternative - intonation. I've always understood ĉu in terms of being a solution to the problem of intonation being used differently by different cultures. (The joke among many Americans for example? Is that Canadians? Say everything? As if it's a question?). Having a word that says clearly "this is a question" solves that problem.

Every language has a way to form a question, and since Esperanto can't use subject-verb inversion, it uses ĉu. Since it's meant to be used as a second language, it's best to use ĉu consistently as an aid to second language speakers (i.e. to everybody.)

Answer 2

Many grammatical features are not absolutely necessary, though you might deem them necessary for added redundancy. Other EO features that are sometimes redundant are the accusative marker (redundant with word order) and adjective agreement (redundant especially when the sentence only has one noun and one adjective). Some redundancy in a language is OK, even desired.

Could EO function without ĉu? Certainly, have a look at Romanian which doesn't normally use any marker for a yes/no question, only intonation in speech:

• Ili venis. = Au venit.
• Ĉu ili venis? = Au venit?

Even so, Romanian has and sometimes uses constructions which stress the fact that this is a yes/no question, such as the conjunction oare, or adding yes/no (da/nu) to the end, or even switching the verb to the first position. Thus there still are several other ways to ask the same question:

• Ĉu ili venis? = Au venit?
• Ĉu ili venis? = Oare au venit?
• Ili venis, ĉu ne? = Au venit, nu?
• Ili venis, ĉu jes? = Au venit, da?
• Ĉu ili venis? = Venit-au? (sounds rather archaic)

Then there are other uses of ĉu, such as in

• Mi ne scias ĉu ili venis.
• Ĉu per trajno, ĉu per buso, ili venis.

Without ĉu we would need other ways to express this.

My take is that ĉu was introduced to keep the word order relatively free, and to avoid use of intonation - different languages use intonation in very different ways, see Mandarin for instance.