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.
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