On Tuesday of last week I wrote that we no longer swoon with surprise when we see a woman slide under an ailing auto "on a mechanic's creeper." On Thursday, in a column about English grammar and usage, I reported that a reader said he "would like to see Washington Post writers to use persuade and convince properly."

a.

learn.

If you saved the errant columns and will be kind enough to clip these two bold-faced lines and insert them in the proper places, I will be much obliged to you. The "a" goes into the word "mechanic's." And "learn" goes between "writers" and "to."

I apologize for handing you a do-it-yourself kit and asking you to correct my typing errors, but the alternative -- to ignore the mistakes -- would be worse. So here are the missing pieces. CUDGELS AT THE READY?

Did Mayor Barry proclaim this to be "Let's Take a Stick to Bill Gold Week"? It seems everybody is after me.

One popular subject has been the question of whether Dec. 31 marked the end of a decade. Your opinions have been almost evenly divided.

Half your letters said a decade did end on Dec. 31, and my comments about decades and centuries was all wrong. The other half chided me for having "failed to discuss this issue and point out the error that is being made so frequently in calling Dec. 31 the end of a decade." Not even I can be guilty of both those charges.

Another current complaint: Women who think I favor drafting women are very angry with me. All I said was that it would have been interesting to let Congress debate the issue.

More than a dozen readers have descended upon me for starting sentences with "And" and "But." No doubt an article repeating this nonsense was published somewhere recently.

It is not incorrect to begin a sentence with "And" or "But." In Roy H. Copperud's "American Usage and Style: The Consensus," we read, "There is no reason why sentences should not begin with 'and,' six authorities agree. And when this is done, 'and' should not be followed by a comma except . . . But the comma is correct to set off a parenthetical element . . ."

As you can see, the author has in one brief paragraph begun one sentence with "And" and another with "But." In fact, he does this throughout the book. Fowler, Bernstein, Bryant, Flesch and Follett all agree.

Last Thursday's column on usage included a sentence that said that if schoolteacher is to be written as one word, hyphenated secondary-school teachers would be preferably to secondary schoolteachers, "which might convey a different meaning than that intended."

Only 11 readers have thus far informed me that I made a mistake and should have written "different from," rather than "different than." Obviously, the bulk of the criticism is still in the mail pipeline and will be delivered to me when the Swift Couriers of the United States Postal Service feel I have been patient long enough to deserve delivery of last week's mail.

In summing up expert opinion on "different than/from," Copperud writes: "Two critics hold that 'different than' (rather than 'different from') is acceptable only in special circumstances. . . . Two critics and Webster accept 'different than' as a standard variant of 'different from' in any context. . . . Bryant finds that 'different from' is usual in such constructions, but adds that 'different than' is standard, which is the consensus."

Keep in mind that there is no single or final authority on the English language. One can merely examine the pronouncements of a handful of students of usage whose work has won the respect of educators, writers, editors, lexicographers and the like. When these eminent authorities disagree among themselves, no amount of discussion among people like you and me will settle the issue or establish that one usage is "right" or another is "wrong."

When expert opinion is unanimous or nearly so, we must accept it. When the experts are divided, each of us must make his own choice. However, a prudent person does not make pompous pronouncements about the rights and wrongs of issues on which even his betters cannot agree. POSTSCRIPT

Speaking of errors, Greg Wiggins notes that our sports pages recently stated that the Soviet hockey team had "35 wins, two draws, and three defects."

I checked with my friends in the sports department and was told, "We noticed the mistake and would have sent down a correction to change 'defects' to 'defeats,' but we were afraid the new line would come up 'defections,' so we decided to quit while we were behind."

I can never figure out when those guys are serious and when they're clowning.