once a day是一天一次,每天一次,强调的是频率；而everyday是每天,强调时间.
例如：
The medicine should be taken once a day after dinner.这药一天一次,晚餐后服用.
The medicine should be taken everyday.这药天天都要吃.
once a day 与everyday有何区别?
once a day是一天一次,每天一次,强调的是频率；而everyday是每天,强调时间.
例如：
The medicine should be taken once a day after dinner.这药一天一次,晚餐后服用.
The medicine should be taken everyday.这药天天都要吃.