One of the things she wrote about was life on a small farm a

句子成分分析尽量简化.首先看One of the things was life...所以用was,还有哪里不明白吗,欢迎追问
再问： 那为什么He is one of my friends who are working hard是用are？！
再答： 注意成分。首先和你说中文吧，我很多朋友都努力工作，他是其中一个。把这句话拆分，He is one of my friends，从句（my friends）who are working hard。也许你会问为什么这里不是one of my friens who are workding hard，但是如果是这样的话意思就不通了。你觉得呢。 至于第一个句子，主系表是我刚才说的One of the things was life...，（that）she wrote about是主语从句