第四题中为什么stay要加s啊.

so he must stay in bed for a week.句中stay为什么不加s He must stay at home.句中的stay要加s吗?为什么? I sometimes stay in bed until lunchtime.为什么sometime要加s During my summer holidays,l stay in Kunmin中的holidays为什么要加s I stay at home on Saturdays 为什么星期六加s stay at home为什么要加at? I think staying at home is really boring 中stay为什么要加ing? stay++up++为什么stay加ing I usually stay home at night.stay home 中home前面为什么不用加at? we'll stay at the best hotels.这句话中的hotels为什么 加s? 第三题friends为什么要加s 为什么有些英语句子中有的单词后面要加S有的不加s