The young man was running in the street very fast

这句话中用is也可以.
但既然用了was那么就表示在描述过去的事情.
加-S或-ES参考如下链接：写的很全
当主语是第三人称单数,且时态为现在时,则动词加-s -es.
例如；I want to go shopping.
she wants to go shopping.
一般来说以sh,ch,th,s结尾的加es
例如；she teaches the piano.
miss →misses
teach →teaches
wish →wishes
此外以y结尾的一般变成ies
fly → flies
具体还是网上去查查资料吧 ,我暂时就写这么多了.