What does "If" indicate in these sentences?

The Supreme Court of Western Australia ruled that it was up to Christian Rossiter, aged 49, to decide if he was to continue to receive medical care (tube feeding) and that his carers had to abide by his wishes.
What does the sentence after the word if mean to you?
If he walked into my life today, I would be happy.
vs
If he walk into my life, I will be happy
If you did not create a user account in the firstboot screens, switch to a console by pressing Ctrl+Alt+F2
vs
If you do not create a user account in the firstboot screens, switch to a console by pressing Ctrl+Alt+F2

If you tried to open home page, it would not get open.
vs
If you try to open home page, it will not open.
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I am a little bit confused about the use of “If” in these sentences. What do you think, “If + simple past vs If + simple present”
Please explain me the difference?