Breakdown of Moramo voditi računa o vremenu.
morati
must
vrijeme
time
o
about / on (dependent on)
voditi računa
to bear in mind, to look after (o + locative)
Questions & Answers about Moramo voditi računa o vremenu.
How does voditi računa translate to "keep an eye on"? I thought račun meant a bill or receipt.
You are right that račun usually means a bill or a receipt. However, the phrase voditi računa o is a very common idiom that means to take care of, pay attention to, or keep in mind. Literally, it translates to "to keep an account of", which is why it takes on this meaning of being mindful or keeping an eye on something.
Why is računa in the genitive case instead of the accusative?
In this fixed expression, računa is in the partitive genitive. Literally, you are keeping "some account" or "a certain amount of account" of something. Many expressions use the genitive for the direct object to imply an indefinite quantity, and this grammar fossilizes in idioms like voditi računa.
Why does the word for time, vrijeme, change to vremenu here?
The preposition o (about, concerning) requires the locative case here. The noun vrijeme (time or weather) is an irregular neuter noun. When it declines into most other cases, it adds an -en- stem before the case ending. So, the locative ending -u is added to vremen-, making it vremenu.
Could I say Moramo paziti na vrijeme instead, and is there a difference?