German word order is very flexible. You can even put object before the verb. And not only subject or object but any part of a sentence keeping in mind the verb has to be at the second position. Example: Mein Urlaub beginnt in zwei Monaten. You can rearrange it in many ways. in zwei monaten "beginnt" (verb at 2nd position) mein Urlaub Another example: Ich liebe dich. You can also say, dich liebe ich. (in English you cannot say, "You love I", but it is possible in German) :) The meaning does not change as it is clear what is subject and what is the object. The reason for the shift is that it is for making stress in a sentence. If you are stressing "dich" and not any other person. Dich liebe ich, aber ihn mag ich. Ihnen wünsche ich ein schönes Wochenende! :)

As far as I know, we can put a verb before the subject as in this sentence: "In zwei Monaten beginnt mein Urlaub." What's the reason for this shift? -> I am sorry that I can't answer the question directly but maybe my comments help you: Will it be correct to say: "Meun Urlaub beginnt in zwei Monaten". -> Yes Or "In zwei Monaten mein Urlaub beginnt." -> No What about these sentences: "Ich wohne in Berlin" - I know this is correct. -> yes "In Berlin wohne ich" - i followed the same logic as in the first sentence anout Urlaub. -> It is not wrong but would get used different. For example you can say "In Berlin wohne ich, aber ich arbeite in Potsdam". (I live in Berlin but I work is in Potsdam). "In Berlin ich wohne" - correct? -> No

German is a "V2" language (http://en.wikipedia.org/wiki/V2_word_order), the (finite part of) the verb always has to be in the second position in a main clause. Your last example has the verb in the third position, so it's wrong.