Rule reminder
Key distinctions: 'dopo che' = after (completed action, indicativo); 'non appena / appena' = as soon as (immediate sequence, indicativo); 'mentre' = while (simultaneous actions, imperfetto); 'ogni volta che' = every time (habitual, indicativo). Choose the conjunction that best fits the meaning of the sentence.
_____ ho sentito la sirena, mi sono fermato.