From: Pasadena, CA Registered: 2009-05-29 Posts: 1039
Obviously they have different on-readings. So, if it's used in a kanji compound (e.g., 乗馬 (じょうば) and 搭載 (とうさい)), it should be easy to figure out which to use.
They can be used for the yamato word のる and related ones like のせる. In this case, as with many other yamato words, you pick the right one depending mainly on the intended meaning, connotation, and to some extent, conventions.
Those yamato words are like the Germanic irregular verbs in English (e.g., "get" and "set") so they have tons of different meanings and tend to be used in idiomatic ways. The striking difference between Japanese yamato words and English words is that the spelling of a word stays the same in English regardless of the meaning you use it for (so "get" is still "get" whether it means "understand" as in "I don't get it!" or "receive" as in "I got a letter from him") while you can spell words differently in Japanese depending on what you mean.
User-friendly J-J dictionaries should list each meaning and usage of a yamato word along with the kanji you're supposed to use. If you don't like monolingual dictionaries or don't think you're ready for them yet, here are some useful rules:
You use 乗 for most of the basic meanings and usages of のる and similar words. So, for example, you use this kanji when it roughly means "ride" or "get on" as in "車に乗っていく (go by car)" and "飛行機に乗る (get on a plane)." This is true for many other senses like "achieve," "enter," and the like, e.g., "He got his company back on track. (自分の会社を軌道に乗せた)."
But there are cases where you always use 載 or should pick the right one carefully according to finer points. Pretty much the only case where 載 is always used is when it means something like "publish" or "put up in written material." Here are a couple examples of this kind:
景気はまだ回復していないと新聞に載っていた (The newspaper says the economy is still in recession.) 研究成果を有名な雑誌に載せる (to publish results in a prestigious journal)
載 is generally preferred like the above case when you mean "inanimate things are put/standing on something," though there are some exceptions in this case. For example, if a book is on a table, it's usually 本がテーブルの上に載っている while if it's a cat, you normally write 猫がテーブルの上に乗っている. This animate vs. inanimate distinction holds true to cases when different translations sound smoother as well. So, when you load stuff on a truck, it's 荷物をトラックに載せる. But when you give your friend a ride, it's 友達を車に乗せる. They're the same word with the same meaning in Japanese, but you spell them differently because of the finer grammatical rule.
By the same token, 載 may work better when you're talking about whether something fits, can be installed or something along those lines. For example, if you're wondering if a new TV you want is too large or too heavy for your TV stand, you might say "このテレビちゃんと台の上に載るかなぁ." Again, if you're talking about people, you use 乗 even if the meaning is exactly the same. Let's say, you have a relatively small car and your friends are all fat and big. And you wonder if everyone can get in the car. In this case, you might say "この車に全員乗るかなぁ." If you're trying to load two gigantic TV sets, it'd be like "この車に全部載るかなぁ." The only difference is the kanji.
There is no such rule like it's 乗 if it's for vehicles. That's simply wrong.
Note that you still use 乗 for other meanings regardless of whether you're talking about animate or inanimate objects. If you remember the example of "getting my company back on track," you use 乗 when you're talking about your company, which isn't a person, animal or living thing. You'll see this very frequently when the meaning is like "agree with," "work well with," and the like. For example, if a particular makeup product agrees with your skin and works well, it's 化粧の乗りがいい. If your voice is very clear and has a distinctive mid frequency so that even a cheap mic can pick it up very well, you have マイクによく乗る声.
If you don't mind an explanation using Japanese, it's simply you use 載 when the word carries the sense of 搭載, 積載, 掲載 or similar compounds and use 乗 otherwise. You'll learn that the max load of a car is 最大積載量 and its equivalent for passengers/seats is 乗車定員. This explanation is kind of a circular argument. But I think it helps to look kanji compounds with 載 or 乗 that shares certain senses with のる and similar yamato words. This is how native speakers choose kanji for pretty much every yamato word that has multiple spellings in kanji.
This is a better explanation if you're already familiar with how transitive and intransitive verbs behave and what kind of meaning each type typically has because this implicitly explains the animate vs. inanimate distinction. But the difference between the kanji has nothing to do with vehicles. Yamato words with 乗 or 載 including のる as an intransitive verb are spelled differently depending on what you mean and, in certain cases, if you're talking about an animate thing.