They begin to understand grammar and complicated sentences.
Many kindergarteners learn to pick up the meaning of advanced language. For example, they may understand when adults are speaking in present, past and future tense, or what adults mean when they say something might or might not happen.
What you should do: Speak to your child like you would speak to an adult. You may be used to speaking in very simple language, but kindergarten can be the time to start breaking that habit.