...:I know here masks (are /is/ served / sold / saw/ seen) (as) one of the most common things
.翻译的大致意思是:我知道在这里,口罩是 /被视为最普通平常的事物,最容易买到、得到的事物。所以①括号里的助动词时态和数的问题:是用are/ is ...
...动名词和不定式的完成式做主语有没有区别。To have made the same mistake twice is unforgivable.Making the same mistake twice is unforgivable.期待曹老师的解答,希望您能有像非谓语动作作宾语一样的观点和总结。万分感谢!