bay指的是海灣，是自然形成的自然現象。
harbor指的是海港，是人類修建的，是人為現象。
例句：
bay
1、Cardiff Bay project is attracting many visitors.
加的夫海灣項目吸引了眾多遊客。
2、The ships in the bay present a beautiful sight.
海灣內的船舶呈現壹派美麗的景象。
3、The bay glittered in the sunshine.
海灣在陽光下閃閃發光。
4、Oil announced that it had struck oil in the Liverpool Bay area of the Irish Sea.
石油公司宣布其已在愛爾蘭海域的利物浦海灣地區開采出石油。
harbor
1、The Harbor was replete with boats.
港口裏滿是船只。
2、They entered the harbor by sailing under false colours.
他們靠欺騙入了港口。
3、There is enough water in the harbor to float a fleet.
海港的水域足夠容納壹個艦隊。
4、The harbor was full of pleasure craft.
港口滿是遊艇。