How do you explain to a foreigner grammatically why Shakespeare wrote something like "until [my death] come"

Answer 1

If the foreigner were a Spanish speaker, then it would be easy. You would just say that he was using the subjunctive after the temporaral preposition "until" as they would do.

For most other foreigners, then you would have to explain that this expression could be expanded into "until such time as my death come" and because of the uncertainty involved as to when that time might be, the subjunctive form of the verb "come" is appropriate.

If the concept of the subjunctive mood is entirely strange to them, tell them that they really don't have to worry, as it is gradually being phased out of the English language and is only used by pedants (like me).

Answer 2

It's the subjunctive mood, which we don't use much in English anymore. Language constantly changes, and this is an example of something that has changed since Shakespeare's time.