If you had told me, I would have used the other route.
If you had told me, I could have used the other route.
Would here denotes a certain consequence of the unreal condition: no question about it, I would have used the other route.
Could denotes a possible consequence of the unreal condition: if you had told me, I might still have taken this route but I would have had the choice of taking the other route.
If you had toldPast Irrealis me, I wouldPresent Irrealis use the other route.
If you tellPresent Realis me, I wouldPresent Irrealis use the other route.
These are not quite impossible, but the circumstances under which either would be acceptable are very unlikely to arise. Ordinarily these forms would be understood as having the tense/mode significances I have noted in superscript, and in most cases the two verbs, the one in the IF clause and the one in the THEN clause, should have the same tense and mode.†
† This is not always the case in conditional clauses involving logical inference; but I do not think either of these sentences can be read as inferential.
In a comment, you said that you knew that "could" is the past tense of "can" and therefore, this is irrelevant to the question.
With the way the question was asked, no context was given, so therefore the comments given were not irrelevant as they are stating facts.
You need to write the question in a way which is clear to others what you are asking in order to get the answer you need.
In the same comment you said
"Could" seems to be the choice in a rhetorical question, e.g. "How
could we possibly work together? You never trust me.". In this case,
"could" has nothing to do with the past or irrealis mood.
Now, when you are making a rhetorical question, you are not actually asking a question. You are talking to the other person (or people) with the aim to produce an effect or to make a statement rather than to elicit information.
A rhetorical question can be expressed in terms intended to persuade or to impress the importance of a "fact", so therefore, rhetorics can be used in the irrealis mood, as you can have a subjunctive mood,
If I loved you, do you think I would do that?
jussive or imperative moods
Paul, can you do your homework now?
as well as other moods in them.
So in answer to your question, the following is used in their relevant ways.
Past tense
How could I concentrate with you breathing down my neck all the time?
Present tense
How can I concentrate with you breathing down my neck all the time?
Rhetorical
You can use either of the two statements but with emphasis on the word can and my neck
How can I concentrate with you breathing down my neck all the time?
Or with emphasis on the words could and my neck with the most effect possible given in the future tense
How could I concentrate with you breathing down my neck all the time?
Best Answer
Neither of your sentences makes it clear that you have given the cake to John.
The usual options would be:
This is making it clear that you might give this cake to John but that you have NOT done so.
Alternatively, if the dog had eaten the cake, you might say:
The cake no longer exists and John certainly didn't get it.
To make it clear that John actually received the only cake you possessed, you would have to rephrase the sentence:
or something similar.