How to record fom a soft synth (bass , drums, piano ?)

What are you trying to record? I hope its MIDI. A little information here would help. Are you using a MIDI keyboard controller? 

have ye hit 'record' on control bar/ on soft synth track ?
track would become red colour, if not -> it's not in recording mode...
 
 

OK what are you playing in? Is it a keyboard on the screen? Are you hitting the various drums shown in the drum synth? 
 
If so that is just for auditioning not for recording. Open up the Piano Roll View and input notes using your mouse. That will be recorded. 
 
Or buy a MIDI keyboard controller to play. It can be hooked up and Sonar will record the MIDI it outputs. 

No its not. Sorry. 

can record also with stepsequencer on soft synth
it's the keyboard icon left up on the Softsynth interface.
specially on drums/bass it can be handy,  can switch/revert to midi recording as desired

To record in real time you need to use a musical midi keyboard or controller .... sorry, it won't record in real time using the mouse.
 
You can use Piano roll or staff view to input the notes with the mouse and that will work..... just not in real time.

As was mentioned, you can record MIDI notes by playing a real keyboard (or MIDI controller) that's connected to Sonar or you can enter the notes manually with the mouse in the Piano Roll View.
 
I don't know of a way to actually record the soft synth in real time, but, once you have the notes in the MIDI track you can "freeze" it using the "freeze" button. This will create an actual wave file of the MIDI track. This is better than real-time-recording because if you need to go back and make changes to the MIDI track you can just unfreeze it, make the changes, and then freeze it again.
 
You can then "Bounce to Clips" which will make it more permanent.
 
Steve