English is a flexible language; lazy even.  We throw sentences together in a pick n' mix fashion, so 'I see the man', or 'the man is seen by me,' or even 'I  went for a walk with the man.'  In essence, the man seems to know his place and obediently follows us as we dot him around a sentence, and we don't give a second thought to that little word 'the', as it stays the same wherever we put it.

And yet, we don't say 'I see he' or 'he sees I', or even 'the man is seen by I', as we instinctively know we need to say 'I see him, he sees me; he is seen by me.'  So what's going on in our own language?  It's grammar, pure and simple.  Suddenly we find our language is not so flexible after all, and even English draws the line somewhere (although we do permit the Wurzels to sing, 'I seez 'ee an' 'ee seez I,' as this is local dialect in the West Country.

German is very different.  It is the Nanny McPhee of all modern languages, rapping you over the knuckles if you misplace a word and tutting furiously if you don't know whether a fish is a he, she or an it before you start.  Let's go back to the man.  If he's the subject of the sentence, in German he's 'der Mann.' Who's the daddy? He's the daddy. But if he's the object of a sentence (as in 'I see the man,') then der Mann changes to den Mann.  Ask yourself 'who sees?' to get the subject (I do, so I am the subject); what do I see? (the man, so he is now the object of the sentence.) The grammar term for subject is called the Nominative. The object is called the Accusative. In English, we also have an indirect object (with the man, to the man), and this is called the Dative. Does it change in German too? Of course; we move from der Mann (nominative) to den Mann (accusative) to dem Mann (dative.)

If all this sounds confusing, don't worry.  Golden rule is, know two things in German about a noun (person, place or thing) before you start - its gender (male, female or neuter, if it is der, die or das) and position in a sentence. And take more interest in English, be aware of the rules that govern our own language as they will help you master others.