As usual, this isn't the whole truth.
Code-switching, or language mixing, is common the world over when bilinguals talk to bilinguals. The usual theory is that there is a universal metagrammar with two rules that control when switches happen:
- switches happen only between words (technically, between free morphemes -- the free morpheme rule)
- switches happen only when the word order of both languages is in alignment (the equivalence rule)
An example of the equivalence rule is switching between adjective and noun. If both languages usually put adjectives before the noun (AN order), or both put the adjective after the noun (NA order), then it is possible; but if the two languages use different orders, then it is not possible.
Switches that violate these rules constitute less than 1% of the data. Note that apparent violations can be induced by simple word borrowings (and bilinguals, unless they are trying to avoid them, make more borrowings than other people).
Here are some examples (accents omitted):
In Example 1, the text violates the free morpheme rule, and was in fact written by an English-French bilingual who had suffered brain damage and was attempting to write French (I have inserted hyphens between morphemes):
- *J'es avec une massio-dial a et except dans le cissuden. We-de main pour la pousse tard being-ig maid mouche was tr-iel mal.
Examples 2, 3, and 4 are typical metagrammatical examples:
- Todos los Mexicanos were riled up.
- So you todavia haven't decided lo que vas a hacer next week.
- J'ai l'impression d'etre back in the country.
Example 5 is metagrammatical, but Example 6 is not, because it violates the equivalence rule:
- J'ai achete an American car.
- *J'ai achete an American voiture.
(because French, as a NA language, would require "une voiture americaine").
The following group of examples shows where switches are possible and not, due to the equivalence rule:
- No se, porque nunca lo use. (pure Spanish)
- I don't know, because I never used it. (pure English)
- No se, porque I never used it (metagrammatical, actually recorded)
- No se, because I never used it (also metagrammatical)
- *No se, porque nunca I used it (unmetagrammatical)
- *I don't se, porque nunca lo use (unmetagrammatical)
- *I don't know, because I never lo use (unmetagrammatical)
The free morpheme rule also forbids switching between clitic and a free morpheme:
- *Yo went to the store.
Switches between verbs in a serial-verb-like construction, though not actually forbidden by the free morpheme rule, are quite infrequent, apparently because the verbs are closely coupled:
- ?They want a venir.
It is also interesting that in immigrant communities, code-switching in ordinary conversation may be socially required: those who speak only the language of the country are perceived as assimilationist, while those who speak only the original language, with no borrowings or code-switching, seem over-formal or show-offs.