So if I have it right:
Past. When I was sick, I took some medicine. Nang.
Present. When (whenever) I am sick, I take medicine. Kapag.
Future. When you arrive at the airport tomorrow, show your passport. Kapag.
@JohnD
For the Present and Future, you are correct.
WHEN/WHENEVER I am sick, I take medicine = KAPAG ako ay may sakit, umiinom ako ng gamot.
WHEN you arrive at the airport tomorrow, show your passport. = KAPAG nasa airport ka na bukas, ipakita mo ang iyong passport. - I think it would be more natural to express it in Filipino as "kapag nasa airport ka na" (when you are already at the airport) than "kapag dumating ka na sa airport" (when you arrive ...). That's because if we are to use "dumating" (arrive), we would understand that more as the point of arrival or the moment you step on any part of the airport.
For the Past, we would still use "kapag" if we mean each time we got sick in the past.
Kapag ako ay NAGKAKASAKIT/Kapag ako ay may sakit, UMIINOM ako ng gamot. = When/Whenever I would GET SICK, I DRINK medicine. ("Kapag ako ay may sakit" literally means "When/Whenever I have an illness".)
We would use "nang" if we are referring only to a single or specific instance of being sick. By default, it would be the last time we were sick. If you are in a conversation, it could be used to mean the time you had the same illness as what the other person has or had. It may also be used when talking about experiencing an illness that normally only happens once in a lifetime, e.g., measles, chicken pox. Otherwise, we would be using "kapag" because they normally recur.
Nang ako ay NAGKASAKIT, UMINOM ako ng gamot. = When I GOT SICK (the last time), I DRANK medicine.
Nang ako ay NAGKASAKIT ng pareho sa sakit mo, UMINOM ako ng gamot. = When I GOT SICK with the same one you have/had, I DRANK medicine.
Nang ako ay NAGKASAKIT ng bulutong, UMINOM ako ng gamot. = When I GOT SICK with chicken pox, I DRANK medicine.