the opposite of boastful is humble.
The opposite of "boast" is "modesty" or "humility." It refers to not bragging or showing off about one's achievements or qualities.
Gloat, boast, honor, or praise. Those are the opposite of humbled.
If you mean by 'advertise' to make public/proclaim/boast... then antonyms (opposite words) might be conceal, withhold, depricate. However there is no antonym for advertise in terms of its commercial use.
The preposition for the word "boast" is "about" or "of." For example, you can boast about your achievements or boast of your accomplishments.
boast
njj
Russell Boast's birth name is Russell John Boast.
Ghost boast
One may use any of several prepositions with the verb to boast, including:boast of (deeds, exploits)boast about (abilities, charcateristics)boast to (anyone listening)boast among (ones peers, or a group)
Robin Boast was born in 1956.
Will Boast has written: 'Power ballads'
"I know you did a great job on your exam, but there is no reason for you to boast about it." Boast means the same thing as bragging.