DOBAR or DOBRA IN SERBIAN

How to DECIDE which ending to use?


DOBAR + hamburger / dan / zec (Sg. nouns ending in a consonant)
DOBRA + kafA / vodA / mamA (Sg. nouns ending in -a)
DOBRO + večE / detE / selO / odelO (Sg. nouns ending in -e or -o)