彼らは激しい議論を始めた。

they (usually male); them

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

violent; furious; tempestuous; extreme; intense; fierce; fervent; vehement; incessant; relentless; precipitous; steep

argument; discussion; dispute; controversy

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to start; to begin; to commence; to initiate; to originate; to open (e.g. a store); to start up; to establish (business. etc.); to start ...; to begin to ...