Why is it not "My mamy chleb."? Later there is a sentence using "my" and "mamy" instead of just "mamy".
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!)
why is sentence "we have a bread" wrong? it wants "we have bread" without "a".
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'
I am a total beginner, and wanted to ask which websites are the best to look up conjugation and declination of Polish verbs/ nouns?
In my opinion, simply the English Wiktionary. Or Polish if you won't find some words, but that happens rather rarely.