What does "If" indicate in these sentences?

If he walked into my life today, I would be happy. (hypothetical, not necessarily expecting something to happen. There is no implication that at some point this will be resolved.)
If he walk into my life, I will be happy (Not grammatical! Subject and verb don’t agree)
If he walks into my life, I will be happy (Implies that at some point in the future I will know whether the event took place, and the issue will be resolved.)

@6rant6 : Feeling sleepy. Forgot to add “s” in my second sentence. :(

“If” indicates a condition. If X then Y.

Different tenses and/or voices may be used inside part X (the condition) or part Y (the result), but some combinations are not meaningful together, or sometimes people think they are saying one thing, but they’re using the wrong tense or voice.

“If he walked into my life today, I would be happy.”
– This is a mis-match of tense and voice. “Walked” is past tense, which also doesn’t match “today”, which is present, so I assume that’s the mistake. It could be:
“If he were to walk into my life today, I would be happy.” – would be a correct match of subjunctive voice about the present. It might happen, and if it did, this is what I say I would feel about it.

“If he walk into my life, I will be happy” is also the wrong form – it looks like you meant the present form “walks”. This is declaring or asserting a fact, rather than using the subjunctive voice. It is simpler and more assertive said this way.

“If you did not create a user account in the firstboot screens, switch to a console by pressing Ctrl+Alt+F2” is ok. It says if you didn’t do something in the past, then do this in the present or future.

“If you do not create a user account in the firstboot screens, switch to a console by pressing Ctrl+Alt+F2” is all in the present, and so suggests that the condition and the resulting suggestion apply to the same time, rather than sequentially one before the other.

“If you tried to open home page, it would not get open.” Again this combined past tense condition with subjunctive voice, and in this context looks wrong. I don’t know what you’re trying to say, so I don’t know how to correct it.

“If you try to open home page, it will not open.” This is not using subjunctive so it is stating a conditional fact or assertion in the present/future tense, which is ok. It is telling someone that if they do something, the result will (not) be that the home page will open.

@Zaku : If you tried to open home page, it would not get opened, instead open.

Open indicates adjective here so I added “ed” after “open”.

Then, does it mean if he or she tried it to open in the past and failed then keep it continue in present or in future?

In the example you give:

”...it was up to Christian Rossiter, aged 49, to decide if he was to continue to receive medical care…”

if refers to an embedded yes/no question.

>> Does Christian Rossiter want to continue to receive medical care? It is up to him to decide.<<

I don’t know if anyone else will agree with me.

@Zaku There’s nothing wrong with
“If he walked into my life today, I would be happy.”

This is what is sometimes called the second conditional: if + simple past + would. The past tense here doesn’t mean the action happened in the past. It means the condition is unreal.

@morphail Oh, quite right. Sorry about that. “If he walked into my life today, I would be happy.” is fine.

@lessonenglish “If you tried to open home page, it would not get opened.” Just makes the result passive voice. Also, I think you missed an article in “If you tried to open [the] home page”. I would tend to write, “If you try to open the home page, it won’t open.” or “When one tries to open the home page, it doesn’t open.”