我曾经也疑问过，现在把当时得到的解释分享给你，希望帮得上忙。


ABOARD vs. ONBOARD/ON-BOARD vs. ON BOARD


For those who are confused about the usage of aboard, onboard/on-board, and on board, it is quite understandable. I will try to make it simple.

There are two slightly differing philosophies for application. Let me clarify. Let's start with the traditional application, after which I will explain the modern application.

Traditional application - you will find that, for instance, the U.S. Navy follows similar guidelines.

aboard - adv. - used in three principal ways.
1. Entering a vessel (etc.): He stepped aboard the ship. Once you've made your way aboard the ship.... While aboard the aircraft....

2. Actions while on a vessel (etc.): You can play basketball aboard the ship. Enjoy dinner while sailing aboard the ship.

3. Nautical[-ish] expressions: Step aboard! Climb aboard! Welcome aboard!

onboard/on-board - adj. - used to modify nouns. This is not an adverb.

Examples:
There are six onboard computers.
Each ship has at least two onboard restaurants.
The spacecraft's onboard staff will guide you to your escape pod.

Note bene:
Onboard is an adjective. It is always placed in front of the noun it modifies. Onboard is not an adverb - this is the most common and egregious misusage. Do not say that someone is onboard a motorcoach/spacecraft/anything. This is an extremely common misapplication.

on board - not its own word/part of speech - i.e. you will not find on board as a defined word in a dictioanry. On board describes that something is aboard a vessel. It's rather more an idiomatic phrase (or as we'll see in the modern application, a quasi-adverb).

Examples:
There are no medical physicians on board.
Smoking is not allowed on board.
The child, thought to have been lost at sea, was discovered to have been on board the whole time.

Note bene:
These are distinct from aboard in that, in this more traditional view, usually nothing will follow on board within the same clause as typically does aboard. Using on board, one is describing the location of something or someone, i.e. something/someone is on board. Aboard modifies an action and is often followed by reference of the vessel, e.g. to step aboard a something. Typically, aboard will not end a sentence where on board functions well, e.g. "There are two whirlpools aboard". Here, on board should be used. If, however, "There are two whirlpools aboard the ship," this is correct.

Modern application - There is a different distinction between aboard and on board. You will find that, for instance, such companies as Disney - a company that, more often than not, strives to apply correct grammar and diction - will generally apply this usage.

- aboard - modifies actions while initially entering the vessel.
- on board - modifies actions that occur while on the vessel; also retains traditional usage.

The distinction moves the second traditional usage (see above under aboard) to on board. Aboard is then reserved for certain nautical expressions, e.g. "Welcome aboard!" or "Climb aboard!", but is mainly applied to actions having to do with initial entrance to a vessel, i.e. stepping/climbing aboard something. On board, then, applies to actions that occur while on the vessel, e.g. "Keep track of your belongings while on board the motorcoach", or "You'll be treated to hors d'oeuvres while on board the ship."

onboard/on-board - retains the traditional application. This is an adjective and is always placed in front of the noun it modifies.

Examples:
Workers repaired the spacecraft's onboard security system.
Onboard lifejackets are standard safety measures on cruise ships.

Note Bene:
Do not be deceived by the number of instances of misapplication of onboard as an adverb - such usage is downright incorrect.