In terms of probability, conditionals in English fall in three categories (some say four):
1- A thing that has 100% chance of happening (Probable/possible)
The structure for this case is
if ...Simple Present.. , ...Will/may(modal in present form not tense) + Infinitive verb...
ex: If he goes to Newyork I will (may etc.) go with him.
2- A thing that has 50% chance of happening (improbable)
If ...Simple Past..., ....would (might etc) + infinitive verb .....
ex: If I won a lottery , I would share it with my friends.
3- A thing that has 0% chance of happening (unreal/impossible)
If ...past perfect...., I would (might etc) present perfect ....
If I had won the lottery , I would have shared it with you.
Now to answer your question
"Is it only when it never happens to change the verb into past tense?"
The answer is : Yes when a thing has no chance of happening you chose from the second or third cases depending on the degree.
I don't know if this bus goes to the shopping mall. But if it went there, I might want to take it
The second case is perfect for the application if you know deep down that there's a chance this bus does not go. So the going maybe improbable but not impossible. Also we are not sure that it is 100% possible.
I don't know if he gets over a cold. But if he got over, he could come join us.
The same thing goes for the cold. It's improbable but not impossible so if he recovered, he could come.
For information consult this page:
http://grammar.ccc.commnet.edu/Grammar/conditional2.htm