Infant research is conducted primarily using the following two paradigms. They make these errors even though they were previously using the same words correctly. Use sentence structure to learn word meaning. There is a universal structure of language.
All languages share certain properties.
There are probably innate mechanisms that guide human language learning. They never learned to use language except to speak a few words. There have been a number of children raised under tortuous conditions who were never exposed to language. The older these children were when found, the less likely they were to learn to use language normally. Deaf children not exposed to language invent their own signs.