"I am a man and she is a woman."
Translation:Ja jestem mężczyzną, a ona jest kobietą.
"a" is the version of "i" that shows contrast. Sometimes both are possible (changing the meaning), but not here. But I cannot think of such a context right now. Anyway, if X is a man and Y is a woman, that's definitely contrast.
Imagine "Ja jestem mężczyzną i on (też) jest mężczyzną". There is no contrast, two men here. The sentence is rather a strange thing to say, but it's correct.
"Ja piję wodę i jem kanapki" - just two things that I do, no contrast.
"Ja piję wodę a ty jesz kanapki" - two people doing different things, contrast.
So "a" is equivalent to "ale" here? (BTW, thank you for your many patient and thorough explanations. I feel like I'm really getting somewhere with this course!)
In a way, "a" is something like "and/but". But it is a translation of "and", in fact. It just shows contrast.
A totally different example:
Rabat jest stolicą Maroka, a nie Algierii! (Rabat is the capital of Morocco, and not Algeria!)
żona =wife. I do not think in English wife=woman. In Polish those words are not interchangeable.
Yes. Putting the first pronoun explicitly shows the contrast more clearly, but only the second one is obligatory.