There are three basic speech styles in Japanese: a plain/informal style, a polite/neutral style, and a formal style.
siblings, parents, close friends
teachers, clients, or superiors
strangers, classmates, colleagues, parents, friends
If you are not sure, or if you’re speaking to strangers or to your classmates or colleagues, it’s safe to use the polite/neutral style, although you may also use the other two styles, depending on the context.
P.S. The words from this lesson and all subsequent ones will be automatically added to your dictionary.