What is a portble part that comes at the beginning of word called?

The portable part that comes at the beginning of a word is called a prefix.

Here are some examples:

* Prefix - "pre" means before

* Unhappy - "un" means not

* Rewrite - "re" means again

Prefixes can change the meaning of a word, often adding a negative or opposite meaning, or specifying time, place, or manner.