How to properly use Interlingua to learn other languages

This applies to many other constructed languages as well (especially Occidental and Latino sine Flexione), but let's go with Interlingua because it's the most often talked about online as a way to familiarize oneself with Romance languages, namely Italian, French, Spanish, Portuguese, and to a lesser extent Romanian and of course Latin. It also has the most detailed dictionaries of the available naturalistic auxiliary languages.

First a quick introduction: Interlingua is one of the so-called "Big Three", and is either the second or third-most used constructed language after Esperanto (Ido is the other one of the three, and it also has a speaker base about the same as that of Interlingua). It is constructed in order to be as easy to read (and hear, but it's mostly written) as possible at first sight, which results in a larger vocabulary to learn but also a more recognizable one for those familiar with one of the source languages. Wikipedia compares the two with this example:

Esperanto	Interlingua	English
sana	san	healthy
sano	sanitate	health
malsana	malade	sick, unhealthy
malsano	maladia	malady, illness, disease
malsanulejo	hospital	hospital
saniĝi	recovrar	to become healthy, recover
sanigi	curar	to make healthy, cure
malsaniĝi	cader malade	to become sick, fall ill

You can see above that the Interlingua words are largely recognizable to an English speaker, but due to the Romance influence a lot of them are only recognizable at first sight to someone who knows a language like French or Spanish. The word for healthy resembles French and others, while the word for health looks like sanity in English but refers here to physical health. Sick is also the same as French and others.

This similarity along with the simple grammar Interlingua uses is why it is often recommended online as an introductory language to Romance languages, a language that is not only simple to learn but similar enough to every other Romance language that they become readable afterwards.

This is true and it is a very good method, but there are two things to say beforehand. They are:

One, be sure to remember that this is a familiarity with vocabulary alone. It will not bring about familiarity with grammatical gender (you still won't know whether a word is masculine, feminine or neuter in other languages most of the time), and verb endings in other Romance languages will still be unrecognizable bits of conjugation tacked on to the end. French  This is actually a good thing, because grammatical gender and grammar differs between the Romance languages and trying to find a common bond between them all there would be impossible, so the best thing to do is to construct a language that uses the elements common to them all (vocabulary) and leaves the grammar as simple as possible. This lets the student spend the majority of his/her time on vocabulary alone.

Two, and this is the most important point (it's the reason why this post is being written), do not simply use the language for conversing with others. Languages by their very nature are huge swaths of territory that even native speakers make scant use of. This means that after studying the language for perhaps a month you will begin to gain a sense of false fluency, because the language is fairly easy to read and a lot of the discussion online is centered on linguistics. It will be tempting after about a month to look at a paragraph like this:

"Introduction al interlinguistica. Interlingua parlate e scribite" es le titulo del curso, arrangiate al universitate del 9 al 27 de novembre per le Departimento de Didactica e Organisation Scholar sub direction de Francisco Salvador Mata e Hermenegildo de la Campa Martínez, ambes diligente autores de publicationes in e super interlingua.

...and think "man, that was easy. I understand this all. I'm done Interlingua."

No you're not. At least not if the reason for learning it is to gain a passive understanding into other Romance languages.

As mentioned, the content you will find online is usually pretty simple and the vocabulary is limited, and this imparts a sense of false fluency. But if you really know a language like Interlingua you should be able to write about and explain some pretty obscure concepts. Diligente autores de publicationes in e super interlingua (diligent authors of publications in and about Interlingua) is easy, but can you say a muddy brassworker? (un latonero fangose / fangose latonero) Can you say bundles of wheat? (fardos de frumento) Can you say a leather belt with spots? (un cinctura de corio con maculas)

It's very unlikely that you'll find a speaker of Interlingua that will be able to write about such subjects without the aid of a dictionary, but the vocabulary is still something the student needs to learn in order to attain this unconscious familiarity with other Romance languages. Laton (brass) is the same in Spanish and in French it's laiton, mud is fango in Italian, and fang in Catalan, a belt is cinturón in Spanish, cintura in Italian and ceinture in French. So these are clearly all words that the student would want to know in order to gain a familiarity with these languages.

So how is this accomplished then? Easy - translate pages and pages of content from one language (probably English) into Interlingua. Conversation with average people will not bring up obscure words like this that must be learned, but a translation from a book will. Pick something copyright-free and with a huge vocabulary like Charles Dickens that will require you to look up the meaning of words you may not even encounter all that often in your native tongue. Continue that practice daily until you have reached a point where you almost never have to refer to a dictionary even to translate works of literature such as these. Only after reaching this point will you have thoroughly utilized Interlingua as a tool to passively understand Romance languages.

How long will this take? From an English-only base and with an hour a day of translation, perhaps six months or so. Not a bad investment for someone who intends to get into Romance languages later on but won't be going abroad in the near future.