Cap Kiwi's remarks are of special interest to me. My oldest granddaughter, just turned 3, has been speaking for some time and is now speaking quite well. I have noticed, however, that while she has learned the past tense, she generally only knows the regular form. So when she wants to make a past out of a strong verb (one with an irregular past, like "write" or "sing") she uses the "-ed" ending. The only irregular past I have heard her use is "saw", so she does know at least one. It will be interesting to see how fast she learns the many other irregular past forms, as well as when she learns the perfect tense. Oddly, she seems to know irregular noun plurals, like "men", "children", etc. This suggests that it's easier to learn irregular forms of nouns than of verbs.