We can thank Noah Webster for simplifying at least some of our spelling. He's one who got rid of the imitation French -our endings, as in labor/labour, honor/honour, color/colour, etc. And Noah also got rid of the French –re ending in words like center/centre, theater/theatre, etc. Centuries ago, we borrowed hundreds of words from the French, but they are now English words which we pronounce in the English manner. And we now spell them in the English way…well, with the exception of Englishmen who stubbornly cling to many of those outdated French spellings.
This leaves us with just three simple endings to worry about: -er, -or, and -ar. But relax. They're easy to explain.
The -ar ending is used mainly with adjectives. For example, familiar, regular, circular, solar, vulgar, etc. However, we must remember that at least sixty commonly used nouns also end in -ar. These include dollar, calendar, molar, and grammar.
The -er ending is used mainly with occupations and persons who carry out an action. For example: swimmer, conjurer, baker, teacher, adviser. We also use the –er ending with single-syllable comparative adjectives: bigger, smaller, faster, slower.
So where does the -or ending fit in? It's used when the root word ends in T or S. Actor, visitor, collector, professor, sponsor, and supervisor are common examples.
For a period during the 18th century, French spellings were fashionable, and a number of words were spelled with -or and even –our: governor/governour, which comes from Old French governoeur, and ambassador, which comes from the French ambassadeur. Councilor and counselor both come from the Old French conseillour, but surveyor comes from surveier and should logically end in -er.
A small group of these words still use -or despite the fact that they describe a person doing something: author and survivor. But one small word has changed; jailor may now be spelled jailer. One warning, however, for you land-lubbers. A sailer is a type of ship, while a sailor is a seaman.