Learn Hardcore Serbian: Ovaj mesec radim više nego prošlog meseca, ali sledeći mesec će biti lakši. - This month I am working more than last month, but next month will be easier.

Ovaj mesec is an example of the accusative of time, which Serbian often uses for expressions like this month, this week, this year, etc.

So:

ovaj mesec = this month / during this month
ovog meseca can also exist, but it usually sounds more like of this month or appears in other grammatical contexts

Here, ovaj mesec radim... means I am working more this month.

Also note that mesec is a masculine inanimate noun, so its nominative and accusative look the same:

nominative: ovaj mesec
accusative: ovaj mesec

That is why the form does not visibly change.

Why is it prošlog meseca after nego?

Prošlog meseca is in the genitive, and here it functions as a time expression meaning last month.

So:

prošlog meseca = last month

The sentence is really comparing the amount of work now with the amount of work last month:

Ovaj mesec radim više nego prošlog meseca
literally: This month I work more than last month

Serbian often uses the genitive without a preposition for expressions of time such as: