Let’s continue the S-Series by talking about beside and besides. I’ve heard a lot of people kick up a fuss over these two, but having thought through their usage, I’m rather surprised. I don’t think a lot of native English speakers really confuse the two forms anymore. The two used to be pretty interchangeable, like toward and towards, but beside generally ceded its non-literal meanings to besides sometime in the 19th century.
Unfortunately, it’s always difficult to get good statistics on the prevalence of different meanings of a word, so I’m basing what I say here what the Oxford English Dictionary, the Merriam-Webster Dictionary of English Usage, and (to the least extent possible) my brain have to say on the matter. I’ve added corpus statistics where possible.
Next to: beside. The most literal meaning is also the primary meaning of beside in modern English. If you’re talking about physical positioning, you definitely want beside; the last attestation of physical-position besides in the OED is from 1440.
(1a) The purple couch beside the road
(1b) I am slightly concerned about the hungry tiger standing beside me.
Idiomatic nearness: beside the point. The “next to” meaning of beside is not limited to physical proximity; there are also idiomatic usages, the most prominent of which is beside the point. The alternative, besides the point, is rarely attested both historically and recently:
Adverb: besides. The OED reports that beside was once a standard adverb, but now it’s become obsolete in most of its adverbial usages and archaic in the rest. So use besides in sentences like:
(2a) Men? Sure, I’ve known lots of them. But I never found one I liked well enough to marry. Besides, I’ve always been busy with my work.
(2b) … lost her social position, job, and husband, and was broke besides. [MWDEU]
If you’re using it as sentence modifier, as in (2a), or as a clear adverb (i.e., without a noun following it (2b)), you probably want besides.
In addition to: besides. Now let’s return to prepositional usages. In modern English, the “in addition to” meaning almost always uses besides:
(3) There was no need to install additional software besides the game itself.
Beside used to be common in this usage, but it seems to have become rare in modern English (although I feel like it may be a common local variant in some places). I’ve found it only rarely in modern American writing, such as “Did Glenn mention anything beside the names I dropped?”, from COCA in 2002.
Other than: besides. A similar usage to the last one, again with besides as the primary modern form. Here I’m talking about using the word to mean something like “except”, as in:
(4a) Having been lost in the forest for days, I began to forget whether I’d ever eaten anything besides acorns.
(4b) Will Los Angeles ever be something besides a “suburban metropolis”?
Summary & caveat
So, in general, beside is used for literal and figurative nearness, and besides takes pretty much everything else (especially all other metaphorical usages).
That said, there is an important point here: each of the suggestions I’ve made above is still a bit fluid, since the distinct usages often didn’t start ossifying until the 19th century. Different usages and different people will vary on how much one form is preferred over the other. Adding an s in (1b) is straight out for me, but dropping the s in (3b) just sounds dialectal. And the further you go back in English, the more the usages will blend together.
Lastly, the promised caveat: it’s very hard to get clear data on the usage patterns of different senses of a word, so while I’m confident that these rules of thumb are accurate for my own idiolect, and fairly confident that they apply to standard American English, I’m not sure how well they reflect non-American Englishes. Use them at your peril.