My question is: Why is "sa" used for "truskawki" and "jest" used for "chleb" ?
truskawka - single form
to jest truskawka. (Just one fruit).
truskawki - plural form or general meaning. More common, because usually nobody talks about single strawberry fruit.
To są truskawki.
chleb - singular or general meaning. Usually if you think about bread you think about single loaf of bread.
Chleby - plural form (a couple of loafs)
To jest chleb. This is bread. (or) This is a loaf of bread
To są chleby. These are loafs of bread.
Some words which are non count noun in English are countable in Polish and opposite way.
Ahh ,that cleared a few things for me ,this stupid book shows picture of a big pile of bread ,but uses the word for one bread. :)
Because bread (chleb) means a single loaf of bread or bread in general.
So you can say;
To jest chleb (This is a loaf of bread) if you point to a single loaf of bread.
To jest chleb (This is bread) if you think about bread in general as a noncountable noun. Like water.
Book uses correct word, but intention was to emphasize kind of food and not a loafs of bread.
if you want to emphasize loafs of bread you should say "to są chleby".
Both sentences "to jest chleb" and "to są chleby" under picture are correct, but meaning of them is different.