I have been using poder in exactly the same way I would use the English meanings can and to be able to. However, as the following equivalent sentences show,poder is sometimes omitted. Whereas in English this omission cannot be done.
No te escucho bien, habla más fuerte. I can't hear you well. Speak louder.
No te oigo bien, habla más alto.
So, my question is, when should one not use poder?
I thought that it would be more intuitive to ask "when not to use..." rather than asking "when to use... or How to use...".(As those might be classed as too broad) However, I don't mind if the title is changes to something more fitting.