myfamily这个单词为什么是第三人称单数

Family有两个意思，作"家庭"时，表示家庭这个整体，是单数概念。做主语时，谓语动词用单数。

My family is very large.

我的家庭是个很大的家庭。（单数）

注意: Family，作"家庭"时，有复数形式，表示几个家庭。

There has been some bad feeling between the two families.

两家人之前有过节。（指两个家庭）

Some families stocked their cellars with food and water.

有些家庭在地窖里储备了食品和水。

Family，作"家人，家庭成员"时，是集合名词，表示复数概念。做主语时，谓语动词用复数。但family一词不用复数形式。

His family came to Los Angeles at the turn of the century.

It had been designed as a family house.