In German, there are some words that can change their plural form in very different ways. This is not always obvious, because many words in other languages simply add an ending and that's it. But in German, there are often changes that you have to memorize.
Take, for example, the word “der Apfel” - apple. In the plural it would be “die Äpfel.” Here it is noticeable that not just an “s” or something similar has been added, but a real change of vowel in the root of the word! This needs to be remembered, otherwise it can easily be confused.
Another interesting example is the word “der Mann” (man). In the plural, it becomes “die Männer”. Here, too, the vowel changes, and you need to remember this in order to build sentences correctly.
There are also words that change not only the vowel, but also the ending. For example, the word “das Buch” (book) in the plural will be “die Bücher”. Here, not only the umlaut is added, but also the ending “er”.
These things can be confusing, especially when learning words and not expecting such changes. But if you get used to it, it's not that hard, the main thing is to pay attention to these things and practice.
These are the peculiarities of the German language - multiple meaning words, changes in meaning due to articles, accents or plural forms. It may seem complicated at first, but if you understand how and when to apply the different rules, everything becomes much easier. The main thing is not to be afraid of mistakes, because it is through them that we learn the language. And it is also important to remember that context always helps you understand what you are talking about.