The word miss as a transitive verb also means "to notice or discover the absence or loss of" such as "When I got off the bus, I missed my watch" (The Free Dictionary).
In this sense, you can say "My watch wad missed" or "My watch got missed". Both the sentences are in the passive. They means that my watch was discovered missing. In formal writing or spoken English, we use " get" instead of "be" to form passives in the structure of get + past participle. For example, my watch got stolen = my watch was stolen.
On the other hand, we use "go" to mean become in the form of go + adjective. For example, he went mad.
In the sentence #2 presented by the OP, the word missing is an adjective. So it's correct grammatically if you say "The keys went missing". I think you can also use " get" in the sense of become i.e. the keys got missing.
So the sentences #2 and 4 are correct grammatically and mean the same thing, whereas the sentence #3 is not correct.
As for the sentence #1, it sounds to be correct, but it doesn't mean the same thing as the #2 and 4. Instead, it means that the keys were discovered missing.
Both instances of it refer to the Soviet Union.
We understand implosion to entail the "departure" or "disappearance" of the USSR—when it went away, the network which it had maintained went away with it.
When a complement or adjunct which would ordinarily follow the verb is 'preposed' (put before the verb, at the beginning of a clause), as with it is in this sentence, it is quite common to 'postpose' the subject (move it after the verb) when the subject is 'heavy' (consists of many words) or represents 'new information' in the discourse.
There is an informative discussion of this subject-dependent inversion in The Cambridge Grammar of the English Language, Chapter 16 §5, pp. 1385-8.
Best Answer
is indicating movement, direction or intention toward you.
Similar to your example, you could say:
which indicates an action (ex. What happened was that I hit my finger with a hammer).
As for your example:
is better, because it is more about describing your current state or condition, not movement (generally the case using to be).