The short story is that's just the way it is:
- W basenie jest woda - In the pool there is a water.
- W basenie nie ma wody - In the pool there is no water.
- W basenie nie jest woda. - That thing in the pool is not water, (but what is it? ) The sentence in Polish can be interpreted diversely as it is considered poorly phrased and the decisive impact on meaning has context
In case of nigdy nie jestem w domu this means I am never at home/There is no place I call home. If you expand the sentence with e.g. kiedy przyjeżdżają kurierzy (when the couriers come), the sentence will be understandable but severely lacking stylistically.
In the longer story I should tell about the relation between have and to be in different languages. There is a theory that in ancient times there was only one verb that had both meanings, which remnants can be seen in this być/ nie mieć. Germans when they form sentences in Perfect tense also for some reason differentiate between verbs with sein and haben.
It's just Genitive of "ja". And the phrase is literally "There never has no me at home", regardless of how strange it sounds in English. If the phrase was "There's no bread at home" it would be "W domu nie ma chleba" (At home there has no bread).
No, "Nie mam się" doesn't make any sense.
What are you talking about is how polish is spoken. Granted the computer might sound a bit off but Poles tend to run sounds together to make certain sound combinations easier to pronounce. It took me a couple visits to hear this. So to our ears it sounds like Nigdy nie mam niew domu....