He joined a union when he started to work.

[Anna2004]

He joined a union when he started to work. ( Why "a" is used instead of "the"?)

Thank you,

Anna

 

[MikeNewYork]

He joined a union when he started to work. ( Why "a" is unsed instead of "the"?)

Thank you,

Anna

Either could be correct, depending on context.

 

[Barb_D]

There are many unions.

 

[emsr2d2]

All we know at the moment is that he joined one of the many unions available to workers. If he started a job where only one union was open to him, then he not only joined a union, but he joined the union.