How do I teach my iPhone new words?

A way you can have your iPhone remember new words is by using text replacement.

To do this open settings ➡️ general ➡️ keyboard ➡️ text replacement.

Once in text replacement, click the + icon in the top right.

Then type the word you want your phone to remember as the phrase.

Lastly, in the top right hit save.

Try using the words more often when typing.