You can compare it to the English words 'mouse' and 'mice'. Here you also see that in the plural the word stem changes a bit.
But a Quenya word doesn't have just two forms, and if you follow the course to the end you will see that one word has many different possible endings.
Words with stem forms have two stems: one for the word when it doesn't have endings, and one for the word when there is an ending attached to it. As an example I'll look at seler (sister). This is the basic form of the word, it is also the form that is found in dictionaries.
But with an ending this word changes to its stem form: selli (sisters), sello (of a sister), sellin (to sisters), ...
Except for the plural, the endings are discussed in lessons 11-17.