Judging by conjugated verb alone, one can tell which grammatical person is used (with 3rd person plural and singular we have to choose between oni/one and on/ona/ono respectively - so here we leave pronouns more often). Economy of the language led to the situation when skipping pronoun is normal and leaving it is usually not. We leave it to underline some contrast etc. and some kind of sentences sound strange without it.
- Mamy chleb! (We have bread!)
- My mamy chleb! Wy nie macie nic! (It is we who have bread! You have nothing!)
As bread is uncountable, like in Polish, there can't be any preposition.
Perhaps in colloquial speech (I am not an English native), like in Polish, it can be accepted ('Mamy dwa chleby'='We have two breads'), but the correct form is 'bread' 'some bread' or 'loaf/loaves of bread' to specify quantity.
And so it is in Polish - 'chleb' is uncountable, so the countable form is 'bochenek chleba'