I'll give a very simple example. I'm thinking I'll say, "I went into the room." In German I can say, "Ich bin in das Zimmer gegangen." "Zimmer" means "Room."

In the middle of the sentence, I realize I should say "I went into my workroom (home office)" because that is more descriptive and says which room I went into. Arbeitsraum = work room. If I say, "Ich bin in das Arbeitsraum gegangen" that is wrong, because the article for "Raum" is "der" (masculine) while the article for Zimmer is "das" (neutral). Since the object of my sentence is in the accusative case, I have to declinate "der" to "den." So I should have said, "Ich bin in den Arbeitsraum gegangen."

As I noted, this is a pretty easy example. It gets much more difficult between cases like dative an genitive, where the declination of "die" (for feminine nouns") is "der" - just like the nominative form of the male article. That is just one of an endless list of errors that change the meaning of the sentence. It doesn't help that there are nouns that can have two different genders!

On language you are (IMO) definitely correct! English has about 5 times as many words as German. Sentence structure in English is much easier and you can swap around words much more easily without changing the meaning of the sentence.

This is why (IMO) English is a very good "international" language. You can make yourself pretty well understood with some basics, and then you can improve meaning and shorten sentences by having a huge vocabulary differentiate when it is better to say "grin" rather than "smile" (for example) or having one word to describe several.