There is a "long-standing custom" to use "they" as a common-gender, common-number pronoun. It's been used for 700 years with antecedents like "everybody", "who", and nouns that can apply to either gender, for instance:

There's not a man I meet but doth salute me
As if I were their well-acquainted friend
- Shakespeare, Comedy of Errors

I would have everybody marry if they can do it properly - Austen, Mansfield Park, 1814

...every fool can do as they're bid - Jonathan Swift, Polite Conversations

A person can't help their birth - Thackeray, Vanity Fair

However, a usage where the antecedent's identity has been established, like "The father told his son to take their football into the garden", seems to be newer and is not yet standard.

AnWulf: according to the OED, clýsan is a borrowing from Late Latin clūsa.

I think they mean the same thing. The OED says of "repetitive": "Characterized by, or of the nature of, repetition; tedious, repetitious."

People in Yorkshire pronounce the vowel of "luck" with /ʊ/, so it sounds like "look". That's not similar to French "u".

I can't let the claim that language is not always changing pass. English has changed in dramatic ways since Chaucer’s time. For one thing, there’s the great vowel shift, where all the long vowels changed in quality and two new diphthongs were created - in words like "mouse" and "mice", which in 1300 would have been pronounced with the vowels in "moose" and "fleece". 

In terms of grammar there were significant changes, like the contraction in the use of the subjunctive, the loss of "be" in the perfect tense of intransitive verbs, and the use of "which" to refer to people. Here is a good overview of the changes in grammar since Shakespeare’s time:
http://www.bartleby.com/224/1504.html

Math and physics are irrelevant *to grammar.* Language is not math.

http://motivatedgrammar.wordpress.com/2011/05/17/is-changing-language-tantamount-to-changing-math/

I'm not in high school. I'd like to see a usage book aimed at adult English writers that proscribes things like "very competent" or "more ideal". But I am skeptical that there are grammar books aimed at high school students that do this. Again, math and physics are irrelevant. I thought we were talking about grammar.

I'm not familiar with any English usage books that prescribe the uses of perfect, competent, ready, ideal, etc. in the way that D.A.Wood describes.

I don't think one can argue that a word should be pronounced a certain way on the basis of how similarly spelled words are pronounced. What about "dive" and "give", "food" and "good", "bead" and "head", etc.

Scottish English has a central vowel /ʉ/ for the GOOSE and FOOT vowels, so for instance "food" and "good". This is close to the French front vowel /y/. This is the closest I'm aware of.