rock & stone
These refer to basically the same physical thing, but they have different connotations.
A rock tends to be rough or unrefined, while a stone tends to be smooth or polished; you throw rocks but skip stones. Rock is in its natural state, while stone has been worked in some way; you visit rock cliffs and the Rocky Mountains, but stone temples and the Rosetta Stone. A cut and polished diamond is a (gem)stone, never a rock (except sometimes in slang). And if it’s really big, it tends to be a rock; think of the Rock of Gibraltar or Ayers Rock or the third rock from the sun. If it’s small, it tends to be a stone; think of kidney stones (or maybe don’t).
Side note: Be careful. “We will rock you” and “We will stone you” mean two very different things.
sermon & homily
These two are often used interchangeably. The Catholic Church and some other Christian churches do seem to make a distinction: A homily is a commentary on Scripture, typically short, while a sermon can be on any topic, and is typically longer, perhaps more formal. Protestants tend to just use sermon for everything.
blond & blonde
Sometimes subtle distinctions help us convey precise meaning. And sometimes, they just piss you off. Blond vs. blonde is firmly in the latter camp, for two reasons. First of all, there’s simply no reason to make a distinction (in spoken language they sound identical and nobody gets confused). And second, nobody can agree on what the distinction should be.
Some people say that, when used as an adjective (e.g., blond hair), it should be blond for men and blonde for women. Others say the adjective should be blond for both men and women. The situation is similar for use as a noun (he’s a blond, she’s a blonde) except that many readers interpret the noun as a woman regardless of spelling.
The upshot is that you’re screwed no matter what, so don’t worry about it too much. (Except for copyeditors, of course, who worry about everything; they should consult their style guides.)
ignite & combust & burn
Ignite means to set something on fire. Once it’s on fire, it’s burning. Typically burn and combust are used to mean the same thing. Some sources say that burning implies a flame, whereas combustion may or may not produce flame, but I’d say that burn is often used in non-fiery situations as well (e.g., chemical burns, radiation burns, burning calories). So I’d say that the biggest difference between combust and burn is that combust sounds more science-y. If anyone knows of a clearer distinction, you’re welcome to leave a comment.
erotica & pornography
Erotica is art that tries, among other aims, to excite sexual arousal. Pornography tries to excite sexual arousal as its sole aim, and it is rarely artistic.
film & movie
Pretty much the same, except that film is 30% fancier.
America & the United States
America comes from Amerigo Vespucci, an early European explorer who wrote about his travels in the “New World.” In its broadest sense, the term refers to the Americas, that is, the continents of North and South America. Anyone living anywhere from Argentina to Canada can rightfully call themselves American.
The United States of America is just that: a nation comprising many states, all of which are in the Americas (specifically, North America). But “the United States of America” is a mouthful, so it gets shortened in a number of ways: the United States, the US, the USA, or simply America.
It’s that last one that’s problematic, because now we have some ambiguity. A bigger problem arises when we ask what to call the residents of the United States of America. Nobody’s going to say we’re “United States of Americans,” and “United Statesians” just sounds dumb. Other alternatives exist, but they all seem odd because nobody uses them, and nobody uses them because they seem odd. Americans we remain.
web & internet
The internet is the global network of computers that’s been around since about the 1970s. The web (short for World Wide Web, which is what “www” stands for) is the subset of the internet that comprises websites and web pages like this one. The web has been around since 1991, when it was created by Tim Berners-Lee.
dentist & orthodontist
An orthodontist is a special type of dentist who has done additional intensive study in, well, orthodontics. Orthodontics deals with problems of alignment in the teeth and jaw. One example of orthodontics would be getting braces. General dentists can do some orthodontic work as well, but it’s not their specialty.