They start using first words, first sentences, first complicated constructions.

他们同时使用第一个词,第一个句子,第一个复杂结构

耶鲁公开课 - 心理学导论课程节选

What this means, if you average it out, is that since children start learning their first words at about their first year of life, they learn about nine new words a day.

如果你将这个数字平均一下,这意味着,儿童从一岁左右,学习第一个词开始,每天学会九个新词

耶鲁公开课 - 心理学导论课程节选