How does the ending of yearned change base word?

The word "yearned" is already the past tense of the verb "yearn". Therefore, it's not a case of "ending" changing the base word, but rather the verb itself already being in its past tense form.

Here's the breakdown:

* Base word: yearn (to have a strong desire or longing for something)

* Past tense: yearned

So, "yearned" is the past tense form of "yearn," and it does not change the base word itself.