Back is an adverb and adverbs are very flexible as to where they come in a sentence but are usually close to the verb. They can usually be omitted (Try your examples without "back".)
Give me my keys back!
Give me back my keys!
Give back to me my keys! (Rather formal and requires particular context)
Give my keys back to me!
Are all possible (as are ...
Achieving this will be not easy.
Achieving this will not be easy.
To negate the main verb, you need to put “not” before the main verb, as in #2.
It is also valid to negate the adjective, as in #1, but this has a slight different meaning and is not as common except in the special case of double negation.
It's too hard to imagine for me
It's too hard to imagine that for me
These are both awkward. You could say:
It's too hard for me to imagine that.
However, it would be better to say:
It's too hard for me to imagine doing that.
I think "extremely hard" or "impossible" would sound better though:
It's impossible for me to imagine doing ...
 the document I sent you ___ .
 the document I sent ____ to you.
Yes: both noun phrases are OK.
In  the covert (hidden) relativised element, marked by the 'gap' notation ___ has "document" as antecedent and is direct object of "sent". "You" is indirect object.
In , again the covert relativised element with "...
Your examples are not complete sentences.
The verb “to send” can be transitive or ditransitive, so both of these are allowed:
I sent the document to you.
I sent you the document.
In both cases, “the document” is the direct object. In the first, “you” is an object of the preposition “to”. In the second, “you” is the indirect object of “sent”. They mean ...
The relevant part of the quote:
Only once had she put the doctrine of non-interference into practice,
This is an example of "subject-auxiliary inversion", where the auxiliary verb ("had") is moved in front of the subject. The reason for the move is that the introductory phrase "only once" has negative force.
Think about the difference between "I think that one kissed him" and "I think he kissed that one".
As a question, "Which one do you think kissed him?" is used where we are asking about him as the object of the action, whereas "Which one do you think he kissed?" is used where we are asking about him as the subject of ...
Can you tell me where did you buy it?
This doesn't work - although you could say "Can you tell me - where did you buy it?" if you were adding a significant pause and making it into two independent clauses. But then both would be direct questions.
Can you tell me where you bought it?
There's also "Can you tell me where you did buy ...
More can indeed often go at different points in a sentence. But what it can't usually do is go inside a constituent of the sentence.
I take it that with others using it is a complement of interact: that complement is a constituent in the structure of the sentence, and cannot usually be split up by a word that is not part of it.
In fact, if I read your second ...
Only the second example is correct.
You can use any of
The reason being,
The reason is,
However, I feel that the sentence being used as an example is incorrect.
I would expect to see eg
The car's wheel ...
The aerial's loading coil ...
The mountain's peak.
https://ell.stackexchange.com/a/47049/628 provides a cleaner answer. Applying it for your example, "dearest" is a predicative adjective so the article is optional. To reveal that it is a predicative adjective you can re-write it as:
I could not face being alone again and losing the person who is dearest to me.
When someone fits in somewhere, they make successful efforts to be acceptable, compatible, etc.
When I joined the team, I tried to fit in by learning everyone's names
and bringing doughnuts.
When another person fits me in, or other people fit me in somewhere, they make efforts or arrangements so that there is a place for me.
We wanted Joan in our team, ...
Both sentences are grammatically correct, but they mean different things.
They wanted me to fit in this world.
That means that they wanted it to be true that I would fit in this world.
They wanted to fit me in this world.
That means that they wanted to take steps to make me fit in this world. Of course, that also means they wanted me to fit.
(In that ...
Yes both positions can be used. In general "that is" flags an explanation or an addition to clear ambiguity, rather than just additional information. For example "This is a transformer, that is, a robot that can turn into a car or a truck." Or, for two Australians talking "I am going to Melbourne, the one in England that is." In ...
such further and other relief as to this Honourable Court may seem just
This may be idiomatic in the legal field, which has retained many words and structures that are not used in modern English, but it would make more sense to the layman if we rearranged things just a bit:
such further and other relief as may seem just to this Honourable Court