I sometimes find myself writing sentences with subordinate clauses where there is number mismatch between the primary and subordinate clauses. For example:
> The oath he swore, those words about serving all the people and not just the favored ones, (was | were) just fluff to him.
The general rule I learned is to ignore subordinate clauses when resolving cases like this. The "outer" sentence, which contains the verb, is "The oath he swore (verb) just fluff to him", and so the correct verb is "was". But the subordinate clause uses a plural (the "words" that make up the oath), so it looks funny to write "was" immediately after.
I am aware that I can rewrite the sentence to avoid this problem. My question isn't about how to write it. My question is: does English grammar have any rules about verb number in such phrases (including, possibly, "don't do it because you cannot win")?
