Smoke!!!

[Imp]

In WinSPww2 it seems like smoke is like a wall of cover

That is after all its purpose so yes it is, a very powerful tool if used correctly. Like most things there are no hard & fast rules to cover it you have to decide what to do based on the situation. I have just used it firstly to screen me from pesky sniper & MG fire & secondly to let me get close enough unhampered to find them.
It is also very handy to get temporary force superiority, screen out half his force & attack the other half for example.

As you are playing someone who is willing to give you feedback I would ask if hes willing to do a rematch after or in a couple of games so you can try & put into practice what you have learnt.
Its probably the fastest way to learn & losing is not a problem so long as you learn something as it increases your chance of future victorys, or at least thats how it worked for me.
Ask him to tell you at the end of the game if he guessed you were there or has a scout or 2 out front watching you, if you moved there he might well have seen it.

You do not have to save shots for opfire but the more a unit fires the easier it is to see it.