Languages change over time. Just ask an older person about the slang they used as a child and compare it to the slang you use. In medieval England, they spoke a version of English called Old English or Anglo-Saxon.
Although Old English is at the root of modern English, when you write it down, it does not look like the English we speak today. In fact, it looks something like this: oðþæt him æghwylc þær ymbsittendra ofer hronrade hyran scolde (Beowulf, Old English).
Anglo-Saxon uses only some of the same letters we use in our alphabet today. Plus, it uses three letters that we no longer use - þ ð æ.
In medieval times, most people did not read or write. Perhaps that is why it is sometimes easier to hear and understand Old English than it is to read it. Still, there are many words in Old English that we do not use today. Here are some examples ...
durst – Dare; to have the necessary boldness or courage for something
whence – From where, e.g., "Whence comest thou?" means "Where do you come from?"
whither – To where, e.g., "Whither thou goest, I'll go." means "Where you go, I will go."
wit – To know, e.g., Canst thou wit what the day shall bring?
