我在公交車裡吃午飯可以說成：I'll have lunch in the bus嗎?有人說 on the bus這個搭配更常見，但是 on the bus，總給我一種在車頂上的感覺，到底應該怎麼理解呢？

The phrase on the bus is quite standard and normal.

on the bus這個短語是一個很標準很普遍使用的搭配。

The use of on does NOT imply that the person is on the roof. Basically, if the speaker is taking a long trip by bus, that person is said to be on the bus during the entire trip.

這裡on的使用，並不是意味著這個人在位置層面的on。基本上，如果一個人說他搭公交車旅行，那麼就應該表達成他在旅行中一直是 on the bus 的。

When a person is boarding a bus, we d typically say the person is getting on a bus. (And at the end of the trip, it would also be quite normal to say that person is getting off the bus.)

當一個人要搭乘公交車，我們就會說這個人正在get on a bus。（並且，當他下車時，我們會用get off the bus來表達）

究竟什麼時候用on什麼時候用in？

If there is no compartment involved, you get ON it. (bicycle, motorcycle, skateboard, etc.) (Note: partial compartments, such as those of convertible cars or open-topped boats, count as compartments; pretty much anything where the vehicle at least partially surrounds you.)

如果交通工具沒有隔間，你就用on。(自行車、摩托車、滑板等)(注:有不是完整隔間的，如敞篷車、敞篷艇等，也算作隔間；幾乎就是有東西遮擋在你周圍就算是了。)

Oddly enough, if the transportation is large enough to allow you to move around freely, you also get ON it. (Bus, train, large boat, passenger plane, etc; anything with an aisle or walkway.)

儘管很奇怪，如果交通工具大到可以讓你自由移動，你也用on。（公共汽車、火車、大型船舶、客機等;任何有過道或走道的東西。)

Otherwise, you almost certainly get IN it. (Car, personal aircraft, canoe, etc.)

除此之外，你就用in。（汽車、私人飛機、獨木舟等）