We have a two year old in the house. She cannot yet speak, yet if you ask her almost anything she understands. Take this glass to the sink. Throw this in the garbage. Do you want water or milk? etc. I would say her understanding is about 90%. Right now today she can say about 10 or 15 words. cama, aqua, leche, vamanos, calle, Jim, nana, mamá, abuela, tio, uvas, pan, aqui, alli, arriba, peepee, poopoo.
So with a vocabulary of so few words she manages to make us understand her. Her sentences are one word. Absolutely no grammar. In the next year her vocabulary is going to explode and she will be talking like crazy, and still no grammar. Learning only from hearing others speak and the corrections she receives.
I am starting to use as much English when speaking to her as I can but it is hard as no one else in the house speaks any English. I want her to be bi-lingual from the start.
I have gotten a little lost but my point is that the most important thing to learning a new language is vocabulary. On a scale of 1 to 100, vocabulary is 100, grammar is down around 10.
Just my two cents.