Hi!

You cannot use if-statements like that. Modelica only use IF-satements for choosing constants to variables, not choosing which equation you want to use.

Think mathematically! 

By inputs I mean flow rate. The flow rate is different in different months. They are just numbers not equations.

I did like "if mod(time,365)>=0 and mod(time,365)<31 then Q=**" as the input for January and so on.

It works. But to cover a whole year 12 ifs must be written. So I'm thinking there must be an easier way to do this. However I'm just a starter...

Can you help me with it if you know any?

Thx

Maybe what you want is a TimeTable for your input.  It takes a table of times and values and outputs the value as the simulation progresses.  You would need to convert months to seconds of the year. 

So if I have the same number for the entire month, I just need to put the smoothness as ConstantSegments? I'll try it! Thanks!

Put in an entry for both the start and end of the month that are the same.  That way it will be constant during the month and change at the end.  If January is 31*24*60*60 seconds long, you could do:
table=[0 10
2678400 10
2678401 20
5097600 20
...

Then for the first 2678400 seconds the output would be 10, then, in 1 second it will change to a value of 20 and hold that value for all of Feb. etc.