There are quite a lot of grammar rules in german. to know the word-endings you need to understand the combination of 3 genders: masculine, feminine, neuter; and 4 cases: nominative, accusative, genitive, dative. For example, the "a" in English can be ein, einen, einem, einer, or eines
But still once you memorized the rules, they aren't that difficult. My english friend said it's possible to know, in a general sense, what a german is talking about, without actually knowing the language. Because a lot of words are very similar to english. (even the swear words... I'm sure other ppl on this board who are in germany would know)