自愿参加活动用英文怎么说

自愿参加活动用英语可以用 "volunteer" 或 "participate voluntarily" 来表示。

"Volunteer" 是一个名词，表示自愿者，也可以用来形容词，表示自愿做某事。例如：

She volunteers at the local animal shelter every weekend. (她每周末都自愿去当地动物收容所工作。)
He volunteered to help with the fundraiser. (他自愿帮忙举办筹款活动。)

"Participate voluntarily" 是动词短语，表示自愿参加活动。例如：

Many people participated voluntarily in the beach clean-up. (许多人自愿参加海滩清理活动。)
She participated voluntarily in the study because she wanted to help with the research. (她自愿参加这项研究，因为她想帮忙做研究。)