Probably the most time-consuming part of studying any language is developing a vocabulary, so it’s always a relief to encounter a familiar word, either from one’s native language or from another language one has already learned. Even if the foreign word is spelled or pronounced differently, or the word it looks like is relatively uncommon (vespero and vespers, for example), it can be a big help.

But not every word that looks familiar is all that close in meaning. If you mistakenly suppose the Spanish word embarazada to mean “embarrassed,” that’s what you’ll probably be when you find out it actually means “pregnant.” If you ask for advice on buying ein Gift in a German store you’re apt to be looked at suspiciously, since that’s the German word for “poison.” On the other hand, in French un poisson is “a fish.”

In language teaching it’s traditional to call these tricky words “false friends,” because when we see or hear one we happily think we’re encountering an old acquaintance in an unexpected place. (I’ve been told that I slightly resemble Paul Gubbins, the screenwriter of the Pasporto course. Once the great Esperanto poet Marjorie Boulton, a friend of his, once looked quite happy to see me at a Universala Kongreso. On getting close enough to get a better look, she said she’d mistaken me for someone else and headed in a more promising direction.)

We sometimes even invent a false friend on the fly in speaking or writing, for example adding an -o to an English noun in the optimistic hope it will prove to be valid Esperanto. Sometimes this works, sometimes the result isn’t Esperanto, and sometimes, unfortunately, it is Esperanto but means something that in context can range from slightly off to incoherent to horrifically embarrassing. I regret to say I know about the last from personal experience, though I’ll avoid being specific.

Something that isn’t exactly a false friend but is at least a closely related source of trouble is translating an expression word-for-word from one language into another. In one of his books (Anguished English, if I recall correctly), Richard Lederer described trying to buy a hot dog from a street vendor in Germany and provoking a good deal of hilarity by ordering a heißes Hund – literally a “hot dog” in the sense of a dog that’s hot, which is not quite what he meant. The actual German expression is, I believe, a true friend for English speakers: ein Hot Dog.

Incidentally, there are some more-or-less parallel pitfalls in grammar as well, though I’m not aware of a name for them. One example is translating “I’m going to the party tomorrow” as Mi iras al la festeto morgaŭ. This seems natural to English speakers because in English we often use the present tense to indicate a current intent with respect to a future action. Esperanto, however, is more logical and uses the future tense to refer to future actions — mi iros in this case.

Unfortunately, we don’t often stop to think about the exact meaning of the grammar we use. Teachers of Esperanto should make a point of specifically warning students about such common mistakes, since otherwise they’re apt to become engrained habits. The same applies to false friends, especially the more subtle ones. If the difference in meaning is large enough — say, using the word muso (“mouse”) for a “moose” — the resulting confusion is likely to make the speaker aware of the mistake.

But often the meanings are close enough that the difference is in practice subtle and the error not at all obvious. In fact, the speaker may accidentally say something that makes sense, even if it doesn’t convey the intended meaning. Of course, if both the speaker and the listeners are native English speakers who share the same misapprehension about the the word in question, there won’t even be a miscommunication.

To take one such subtle example that’s in practice a very common mistake, the Esperanto equivalent of the English word type — in the sense of a classification, as in a type of vehicle — is not tipo, but speco.

In Esperanto, a tipo is a “model” or a “characteristic or ideal example.” To quote the Reta Vortaro, tipo means:

(1) Ideala modelo prezentanta altagrade en si la karakterajn ecojn de iu speco da personoj aŭ objektoj … (2) Modela peco laŭ kiu oni fabrikas similajn pecojn; speciale la plumbaj prestipoj kiajn oni iam uzis en presado.”

In other words, a tipo is, sensibly enough, something that’s tipa. To confuse things a little further, the English word type does sometimes have the Esperanto meaning, as for example in the root of the word archetype. Similarly, argumento is equivalent to one sense of the cognate English word (in that of a logical “argument” in favor of some point), but argumento does not mean “dispute.” Likewise adulta means (and looks a little like) “adulterous,” but is dangerously similar to adult. (Watch how you praise someone for acting grown up!)

Other fairly common false friends include aktuala (“current, topical”), akurata (“prompt, punctual”), balanci (“to rock or swing”), demandi (“to ask,” not postuli), oportuno (something convenient; “opportunity” is usually best translated okazo), frazo (“sentence”), novelo (“short story”), pretendi (“to claim,” not ŝajnigi or preteksti), kuraci (“to treat medically”, “to cure” is resanigi), sorto (“fate”, not “sort” or “type,” that again is speco), kontroli (“to check or monitor”; “to control” is regi), and, of course, farti, possibly the favorite verb in Esperanto of 12-year-old English-speakers. (One suspects the expression mi petas is similarly popular among the 12-year-olds of France.)

A useful practice for anyone using Esperanto (or for that matter their native tongue) is to get into the habit of occasionally looking words up in a dictionary. When reading a magazine article I’ll sometimes (probably not as often as I should) underline a word or otherwise make note of it, then in some odd moment look it up to see if it really does mean what I think it does.

You can find a number of useful lists of Esperanto false friends online, including one from the Esperanto Association of Ireland.

Many times a false friend in Esperanto appears in other languages as well, usually with a meaning closer to that of Esperanto. For example, the German aktuell and the French actuel mean essentially the same as the Esperanto aktuala, but not the English actual. This is another way in which learning Esperanto helps students when they study other languages, both by making them conscious of the existence of false friends and by introducing them to words with cognates in other major European languages.

But note that speakers of other languages do encounter false friends in Esperanto. Wikibooks has a page listing examples in several languages.