he; boyfriend

a piece of research, a study, investigation

to research, to study

to go in; to come in / 要る: to be necessary, to be required

to enter, go into, get into a place or a thing / to join a group or organization / to be contained (in), to be included (in) / to be a member of a group, set, category

period / time / date / term

(television show) season / period (of time) / geological age

animal

epoch

everything