Learn Hardcore Serbian: On radi tamo.

Serbian verbs usually show the person clearly, so Radi tamo. is a perfectly natural sentence and still means He works there / He is working there.

You keep on when:

you want to emphasize he
you want to contrast him with someone else
you need to make the subject especially clear

For example:

Radi tamo. = He works there.
On radi tamo. = He works there / He works there.

What form is radi?

Radi is the 3rd person singular present tense of the verb raditi.

So the verb pattern is:

ja radim = I work
ti radiš = you work
on/ona/ono radi = he/she/it works

In this sentence, on radi matches he works.

Can On radi tamo. mean both He works there and He is working there?

Yes. Serbian does not have a separate tense like English is working for most everyday situations.

So can mean: