英语中 'st(有意思)的

莎士比亚十四行（the Sonnets）诗第一首诗第六行
即：Feed'st thy light's flame with self-substantial fuel
（...用自身作燃料培养那眼里的火焰...）

Feed'st thy light's flame = provides sustenance for the flame that gives light. Candles, tapers and oil lamps were the only source of light in Shakespeare's day.
self-substantial fuel = fuel from its own body. Although the general sense of this line seems to be that of a fire or lamp burning up fuel, there are difficulties of interpretation. After all, how is a candle meant to feed itself, other than with itself? The suggestion is that the fuel should be renewable. It implies a criticism of the youth, who is intent on devouring himself and his future hope.

早期现代英语中
单数要用 thou “你（主格）”，thee “你（宾格）”，thy / thine “你的”
复数要用 ye “你们（主格）”，you “你们（宾格）”，your “你们的”
而相应地，第二人称单数的动词要加 st。Thou lovest “你爱”。

古代拼写有省掉某些字母甚至音节的习惯，所以thou lov'st 也有写。mak'st, giv'st, tak'st 就是 makest, givest, takest 的缩写。
现代英语的只留下了you，而ye, thou, thee 包括动词第二人称单数都不再使用了。
然后说“古英语”，这是公元五世纪到十一世纪间的英语。
之后受到法语影响，英语进入中古英语时期，再后来才是早期现代英语、现代英语。