Learn English – the difference between “if he has got” and “if he got”

Reasonable means that whatever decision was made is appropriate given the particular circumstances, and most would regard it as so.

Justified means that whatever decision was made is done for a good reason, in the interest of fairness.

There are times the words could be used interchangeably, and contexts where the difference is subtle. It all depends on the nature of the request or issue at hand.

I might use "justified" if I was angry about something, and was acting in response:

Was it polite to skip their wedding after they insulted me like that? No, but I think my decision was justified.

I might use "reasonable" if I think everyone would agree that there was nothing else that could be done:

Was I happy about missing their wedding so that I could go to my mother's funeral? No, but I think my decision was reasonable.

But there are contexts where either word will work just fine:

You said that you'd deliver the cake before noon, but it didn't get here until 3 o'clock. I think my request for a refund is reasonable/justified.

In that sentence, reasonable would work because most people would agree that a refund would be appropriate given the circumstances, and justified would work because the request is fair.

Virtually means "almost (in respect to degree or strength)" or "almost completely".

In your first pair, the difference is very small:

This mission is almost impossible to achieve.

This mission is virtually impossible to achieve.

Here, I think virtually is a stronger choice, so the second sentence simply means the mission is more difficult than the first. Both sentences mean basically the same thing.

Your second pair has a much bigger difference:

He was almost killed in that accident.

He was virtually killed in that accident.

Here, the first sentence can mean either:

• He was almost hurt in an accident, but he wasn't hurt at all. (For example, his car almost fell off a cliff, but it didn't fall. He got out safely.)

• He was hurt very badly in an accident. He almost died from his injuries.

On the other hand, the second sentence (with virtually) can mean only the second sense. He was hurt badly and was close to dying.

Perhaps a good rule is to imagine that the adjective modified by virtually really is true, but then take a tiny step back:

He was killed in that accident. He's dead.

He was virtually killed in that accident. He's been in the hospital for a month.