Recently, a father wrote in about his daughter’s French. I think there’s enough detail included even if you do not know this particular person (he is one of the moderators of flteach).
“French kids learn to use the subjunctive early, but progressively.
IOW Basic & very common forms are used very early (Il faut que tu
viennes.) but less common forms are acquired later. My daughter’s
French is good for a child living in the US and not attending school
in France, but tonight she surprised us & the friends we were
visiting in La Rochelle. We had about 1/2 hour to drive home for
dinner and were late (as is often the case) and the sky was
threatening. Emilie came in and told us “On va partir avant qu’il
pleuve!” We all looked at each other because none of us expected her
to get this particular subjunctive right. Naturally, she has never
been taught the subjunctive or anything about it, and this form seems
to uncommon for her to have picked it up yet.
I chalk it up to the amount of time she has spent with kids her own
In any case, my students often ask about how & when French kids learn
the subjunctive. Here’s an example of the use of a less than common
form by an 8 year old whose English is the stronger language.”
For those who do not know French, the indicative form is pleut. pr. ploe; the subjunctive cited is pr. ploev. oe = mid vowel like German o umlaut.
This experience is similar to those experienced by many of us who use a lot of CI. Students come up with forms we are not sure they ever heard or read; we cetainly didn’t “teach” them.
I find this sort of thing to be as close to proof that CI works as you are going to find.