BILL,together with his friend Ted , were sailing on the sea.为什么用WAS

together with 是就远原则，谓语动词和Bill保持一致，这个Be动词的单复数要看Bill，而不是两个一起看
所谓就近/就远原则是指句子的主语结构很复杂时,如何判断句子谓语的数的问题.
1. 就近原则: there/here+be/系动词，either...or...，neither...nor...，not only...but(also)...，not...but...，分数/百分数修饰名词（population类外），表许多的名词（除了quantity）
2. 就远原则：as well as, (together/along) with, rather than, except/besides/but/including
如： Not only you but also I am right.
2/3 of the earth is covered by water.
Many a student in our school has never been abroad.
He rather than I is right.
Nobody but two students is in the classroom.