The verb to fear
is here in second person feminine singular form. Thus, the Lord is referring to the city of Jerusalem. Support for this is found in the preceding verses. You will fear
agrees in gender (feminine) and number (singular) with the city
in Zephaniah 3:1, which is modified by the feminine singular active participle oppressing
and is a reference to Jerusalem. The verbs in Zephaniah 3:2 are also in the feminine singular form. The possessive pronoun her
appears three times in Zephaniah 3:3, twice in Zephaniah 3:4, and once in Zephaniah 3:5, each time agreeing in gender (feminine) and number (singular) with that of you shall fear
in Zephaniah 3:7.
7 I said, ‘Surely you will fear me; you will accept correction. Then your dwelling would not be cut off according to all that I have appointed against you.’ But all the more they were eager to make all their deeds corrupt.