You might have seen such a picture: a mother speaks to her child in Russian (or whatever heritage language), and the child answers in English. The simple definition of receptive bilinguals is “people who understand but do not speak”. This is reality for quite a large number of heritage speakers. After testing Labrador Inuktitut receptive bilinguals, I can say that there are two myths about them. One, that they understand everything. The other, that they cannot speak the heritage language at all.
There is definitely a huge difference between their speaking and listening abilities. But even in listening, they miss some parts. Of course, they encounter unfamiliar words, but they also have problems with grammar. They might not be able to get the difference between sentences like The boy is pushing a girl and The boy is being pushed by a girl, even if they know all the words. Fluent speakers might not be able to explain the difference, but they imagine the right pictures when hearing these sentences. Without our implicit grammatical knowledge, we wouldn’t know how words in a sentence relate to each other, and be left to guess, unless our knowledge of the world helps us. Receptive bilinguals still have at least the most basic grammatical knowledge, but some pieces are completely missing, and for others, they have trouble connecting, for example, a suffix and what it is used for. Because of that, it is difficult for them to build a sentence when they are speaking – they, for example, know that the verb needs an ending but cannot remember which ending; in languages that have cases, they may not know what case suffix a noun needs, and so on. If they try to speak, their speech may be very slow, they may make errors, and may give up and switch to English. That’s why they avoid speaking in the heritage language, but it’s not true that they cannot speak at all.