The term irregularities or exceptions in Interlingua refers to deviations from the logical rules in a few grammatical constructions in the international auxiliary language Interlingua. These oddities are a part of the standard grammar. These special cases have crept into the language as a result of the effort to keep it naturalistic. Most of these irregularities also exist in Interlingua's source languages; English, French, Italian, Spanish, Portuguese, and to a lesser extent German and Russian. This feature of the language makes Interlingua more familiar to the speakers of source languages. And at the same time, it makes the language more difficult for others.
The speakers of the source languages do not perceive all deviations as irregular. For instance, Interlingua has three different words for English "am" (so), "is" (es) and "are" (son). While most English speakers will not find any thing abnormal about it, speakers of a few other languages may find the use of three words to express the concept of 'simple present' as unnecessary.
Interlingua is notable in the sense that unlike most auxiliary languages, that seek to minimise or eliminate any irregular aspects, Interlingua takes a flexible approach. It is mandatory to use certain exceptions in Interlingua while others have been kept optional.
Interlingua does not have a 'one letter one sound' orthography. As in English, several letters can be pronounced in different ways; depending on where they are in a word. For instance, the letter C can be pronounced as either /k/ (canto) or /ts/ (cento). Here is a list of other mandatory exceptions in pronunciation:
| Letter / Digraph | Possible Pronunciations[1] | Rules | Examples | |
|---|---|---|---|---|
| c | /k/ /ts/ (or /s/) | /k/ when c is followed by a, o, u or any consonant' /ts/ (or /s/) when e, i or y come after c | camera, crear acido, Cinderella | |
| ch | /k/ /tʃ/ /ʃ/ | like /k/ in words of Greek origin /tʃ/ only in a few words (very rare) /ʃ/ in several words that come from the French | cholera, chrome Chile, cochi machine, chef | |
| h | /h/ silent | silent after r and t | horologio rhetoria | |
| rh | /r/ | pronounced as the "r" in Spanish caro | rhetorica, rheumatic | |
| sh | /ʃ/ | pronounced as "sh" in English | Shakespeare | |
| th | /t/ | always pronounced like /t/ | theatro | |
| ph | /f/ | always pronounced like /f/ | photographia, photosynthese | |
| t | /t/ /ts/ | pronounced as /t/ except when followed by an i and a second vowel in an unstressed syllable, in which case it is realized as /ts/ | tourista creation | |
| u | /u/ /w/ | /u/ except when unstressed before a vowel, in which case it can be realized as /w/ | luna, plural persuader, superflue | |
| y | /j/ /i/ | /j/ when unstressed before vowels other like /i/ | Yugoslavia, yoga tyranno, typo |
Besides, there are also unassimilated loan words that retain their original pronunciation and spelling.
Diacritics are permitted when they do not influence the pronunciation of the word borrowed into Interlingua. Common examples of such words are radios Röntgen (X rays) and kümmel.
Just as in English, where I am is usually contracted to I'm and he is to he's, such contractions are also found in Interlingua and these two are compulsory to observe:
| Words | Contraction | Example | |
|---|---|---|---|
| de (of) + le (the) | del | del matre (of the mother) | |
| a (to) + le (the) | al | al luna (to the moon) |
Plurals can be formed in three different ways depending on the ending of a noun.
| Ending | Add | Example | |
|---|---|---|---|
| ends in a consonant other than c | -es | conversation - conversationes pais - paises | |
| ends in c | -hes | roc - roches choc - choches | |
| ends in a vowel | -s | radio - radios academia - academias |
There are also irregular plurals that occur in loan words. The common ones are tests (from 'test'), addenda (from 'addendum') and lieder (from 'lied').
| Numbers | Logical Name | Name used | |
|---|---|---|---|
| 10 | unanta | dece | |
| 20 | duoanta | vinti | |
| 30 | tresanta | trenta | |
| 40 | quatranta | quaranta | |
| 13 | tresdece | tredece | |
| 14 | quatrodece | quattuordece | |
| 15 | cinquedece | quindece | |
| 16 | sexdece | sedece |
Note. These forms are rarely used.