Ip Kin Yuen observes that except in a very natural linguistic environment, "for instance where the father is English and the mother Chinese," language learning theory suggests that a foreign language is best studied on the basis of a "mother tongue," especially in adulthood. The firmer the grounding in the mother tongue, the more successful study of the foreign language is likely to be.
葉建源指出，除非是有極為自然的語言環境（如父親是英國人，母親是中國人），否則，從語言學習理論來看，外語學習應還是以「母語」為基礎，尤其是成年以後的學習，母語基礎越穩固，外語能力越強。