It's wrong to use "ihn" here. See http://www.thegermanprofessor.com/wort-der-woche-german-als.
"As a general rule, the phrase that als introduces is in the same case as the phrase it refers back to." This is true for all the various uses of als.
Because the sentence is really "Ich schwimme besser als schwimt er." Or in English: "I swim better than he swims." As the best high-school English teacher in the world, Mary Louise Thomas, would say: "Did you truly mean to say 'I swim better than him swims'?" The second use of the verb is implied, but informs the grammar anyway.
I'm afraid Mary Louise is a bit off here:
Both "better than he" and "better than him" are correct. The reason is that "than" can be either a conjunction or a preposition. The final verb "swims" is only implied when you choose to use "than" as a conjunction.
Conjunction: "I swim better than he (swims)." Here we are using "than" as a subordinating conjuction.
Prepostion: "I swim better than him." Here we are using "than" as a preposition. (In this case, the phrase cannot be completed with "swims.")
Here is an authoritative source:
Merriam uses "older than I" and "older than me" to exemplify these two correct uses of "than."
Here is another relevant link:
Yes, both of those are true. But the specific issue I was addressing was the oft heard assertion that "____-er than me/him/her" is incorrect in English. In the context of comparisons (in English), I think the object-pronoun + preposition construction is probably more common than the subject-pronoun + conjunction version -- especially in spoken English. People who prefer the latter sometimes look down on those who don't, and claim to hold the grammatical high ground.
In English, as in French, we use the accusative in this case ("him"), but in most European languages "he" would be correct. This was historically the case in English as well, and there are prescriptionist grammarians who still insist we should. Most English speakers are unaware of what prescriptivist grammarians have to say, though, so they just have to learn this by rote when learning languages like German.
I wouldn't try to use that as a rule of thumb, analogkid.
als (in this situation) is a "subordinating conjunction". So, as furrykef alludes, it doesn't really force a case either way. The choice between er (Nom), ihn (Akk), or ihm (Dat) is determined by how "he" is used within the clause introduced by als.
In this situation, the idea being expressed is "My ability to swim is better than his ability to swim", i.e., "I swim better than
he [does]." The "does" is understood. The "better than" is a conjunction. In a sense, you have two sentences: "I swim" and "he swims", and you are comparing the two.
The importance of the choice of case in these kinds of sentences where words are dropped/understood can be illustrated thusly:
- I like her more than him.
- I like her more than he.
The first is really: I like her more than I like him.
The second is really: I like her more than he likes her.
I would expect "als" to take whichever case makes sense. Here "er" is being contrasted with "ich", which is nominative, so "er" is also nominative (compare English "I swim better than he" -- though in English this is considered pedantic usage). If it were being contrasted with an object in the accusative, as in "I like him better than her" (never "better than she", unless you mean "better than she likes him") I'd expect it to be accusative.
That isn't a great translation of the German sentence, partly because there is a simple, direct translation. More importantly, it has a somewhat different meaning. Consider:
Both Mai Lin and Diego are better at swimming than I am. But I swim better than Mai Lin because she doesn't try very hard, and I swim better than Diego because he is hustling you. He doesn't want you to know how good he really is.