How to move one word left in the vi editor

Use b to move back one word.

Use w to move forward one word.

And here is a cheat sheet that might be useful for you:

Source: Graphical vi-vim Cheat Sheet and Tutorial

It's b.

You can see other motions here:

Vim documentation: motion, 4. Word motions

Generally a Vim command consists of:

count action motion

Where:

count is number of times you want it to execute. The default is 1.

action is obviously an action: d for delete, c for change, default is empty, and it means simply move.

motion is the direction. You got that already.

In addition to the b movement mentioned in the other answers, another movement which may be interesting is ge.

It brings you to the last letter of the word on the left. When b is considered as the opposite of w, ge can be considered as the opposite of e which brings you to the end of the current word.

Also note that all of those word-wise movement have a WORD-wise equivalent: W, B, E and gE which are "faster". For the difference between words and WORDS see: :h word.

Yes, you can use "b" to backforward a word, and in advance, "2b" to move back two words.