not if adjective is alone, without noun.
be adjective always translates to być+ adjective in nominative, no "to", no "instrumental"
also if anything I am sad man would be "jestem smutnym mężczyzną", smutnego is masc/neuter genitive, masc animated accusative.
Dziś jestem wesoły/radosny
"cieszyć się" is a verb. And for some reason "Dziś się cieszę", sounds weird.
That works, but "Today I am sad" gives exactly the same contrast as the Polish sentence: I may be happy usually, but today I am sad.
But "Today I am sad" isn't proper English :) If you break the SVOMPT rule there must be a comma "Today, I'm sad" but it's better not to break the rule.* There are similar cases in the German course. However they allow to use to proper English word order instead of the German word order.
- The easiest way to piss of my former neurotic English teacher (British native speaker) was to break the SVOMPT rule :) Believe me, I had really hard time to internalize this rule as it feels quite unnatural to me.