为什么说动词be是 to be?

为什么说动词be是 to be?
最近看了一段话：
The Forms of “To Be”
The Greek sea god,Proteus,was (like the sea) capable of changing form in an instant.In order to get any decent information out of him,you had to grab him and hold on tight while he went through his various forms — lion,wild boar,snake,tree,running stream — it wasn't easy.The verb “To be” is said to be the most protean of the English language,constantly changing form,sometimes without much of a discernible pattern.Considering that we use it so often,it is really too bad that the verb “To be” has to be the most irregular,slippery verb in the language.
为什么每次提到“be”不直接说“be”?而一提be就是“to be”,看了好多都这样.
见网页：|
"to work" "to write"为什么这样表示？