One thing is that when you begin a sentence, to get the grammar correct sometimes you need to know what comes at the end of the sentence, along with understanding the context in which nouns are used.
For example "the blue dog". In German if you said "I pat the blue dog" it would actually change to something like "I pat then bluen dog", whereas "I am the blue dog" wouldn't have the "-n" endings on the word "the" and "blue".
Once you change the word "dog" for "cat" then the endings on the words "the" and "blue" change, because every noun in German has a different gender (masculine, feminine, neuter), and the words endings in a sentence can change based on the particular gender and context they're used in.
Anyway, this is a long way of saying - you need to know what you're talking about BEFORE you begin!
In English we have the luxury of kind of making it up as we go. If we decide last minute that we're not patting a blue dog, but are patting a blue cat then that's simple, we just change the noun at the end once we get to it. In German you need to have that noun info upfront, otherwise you might not get the grammar correct!