It has been more than fifty days since I took part in thean activity of building a civilized city.

You haven't yet introduced the topic, so you should use a/an initially instead of the.

I hope our city can pass this inspection and earn acceptance.

Keeping cities clean is relatively easy, but getting everyone civilizedto act politely is harder.

You could use civilized instead of politely too. I think this sounds slightly better though.

A clean environment plus civilized citizens equals happiness and joy.

Another way to say this is:
A clean environment and civilized citizens creates happiness and joy.

I hope our city can pass this inspection and gain acceptance.

Keeping cities clean is relatively easy, but getting everyone to act civilized is harder.