ひとりの警官が走って過ぎていった。

one person; alone; unmarried; solitary

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

policeman

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

to run; to travel (movement of vehicles); to drive; to hurry to; to retreat (from battle); to take flight; to run away from home; to elope; to tend heavily toward