The verb बजना means “to ring”, “to clang”, “to ding”, “to chime”, etc.
Here are a few examples:
घंटी बज रही है – “The bell is ringing”
तुम्हारा फोन बज रहा है – “Your phone is ringing”
It has a transitive counterpart, बजाना.
सयाली ने घंटी बजा दी – “Sayali rang the bell”
The transitive verb बजाना can be used to describe playing an instrument: राज तबला बजाता है – “Raj plays the tabla”.
The verb बजना is used to discuss the time of day. It literally refers to the ringing or chiming of a clock. Thus, it is very similar to the English idiom “o’clock”, which refers to “chimes o’ (of) the clock”.
Here are a few examples:
कितने बज रहे हैं – “What time is it?” (literally,”How many are chiming”)
कितने बज गए – “What time is it?” (literally, “How many have chimed”)
कितने बजे हैं – “What time is it?” (literally, “How many have chimed”)
Note that each of the preceding sentences was translated the same way. In Hindi, several different grammatical options are available, all of which have the same basic meaning.
Here are a few more examples:
दस बजने वाले हैं – “It’s almost 10 o’clock” / “It’s about to be 10 o’clock”
साढ़े सात बज रहे हैं – “It’s 7:30”
पांच बज चुके हैं – “It’s already 5 o’clock”
एक बज रहा है – “It’s one o’clock” (note the singular verb)