Sorry if this sounds stupid. But I CAN'T understand this:
How do I know if a verb (like 行) will end with ru or u in the dictionary form? I know that 行 will be 行く in the dictionary form. But why? Shouldn't it end with "る"? I'm using an app that only shows the base kanji without ru or ku/gu/su/mu[...] at the end. I would be glad if someone answered me. Thx!
You know the ending of the verb in plain/dictionary form by knowing the word.
The kanji is not the word itself.
行 alone can have multiple readings: as a verb おこなう and いく are two examples. Using kanji, these would be written as [行]{おこな}う and [行]{い}く respectively. You need to know the underlying reading; that is, the word itself. 行 also has additional readings such as ギョウ and コウ. The distinction between these are that おこなう and いく are called kun-readings and ギョウ and コウ are called on-readings (this becomes quite a rabbit hole if you want to go down it).
If you consider the following verbs, とる, あう, and とぶ they have three different endings for their dictionary form.
If you consider the kanji for these, then you might have [取]{と}る, [会]{あ}う, and [飛]{と}ぶ. (I say might because the meaning of the word will largely determine the kanji used such as [撮]{と}る, [合]{あ}う, and [跳]{と}ぶ. I've just chosen three standard readings you would learn as a beginner in Japanese.)
The portion of the word that is written following/outside of the kanji is called おくりがな (okurigana). This is just something you need to memorize. It's often just the inflected portion of the word. But, there are times when the inflected portion is not sufficient to clue you in to the underlying reading. Another rabbit hole with lots of apparent exceptions which with sufficient practice and understanding matter less and less.
You should try to find a dictionary that lists out the different readings of the characters. Not sure what to say about your app.
This is a typical entry from a dictionary for elementary school children. From this we can see that 行 is taught in 2nd grade. It lists out the five most common readings, splits them up according to on- and kun- readings and indicates okurigana by using a lighter font.
There's actually a lot other information in this image. For example, above the entry for 行 is the total stroke count. Below it, it lists the radical and the stroke count that remains after considering the radical. (Here 行 is its own radical.)
