The words "grim" and "gloomy" are somewhat vague, so there isn't an exact distinction between them that most speakers would understand. As usual, the best way to understand these subtleties is to take note of their primary meanings (which are sometimes rare or partly forgotten), and follow how people reasonably extend those meanings to describe different situations.
The noun "gloom" primarily means the dull or dark lighting that you find at twilight, on a very cloudy day, or within a shadow—with the connotation of the melancholy or despondent emotion that people often feel in places with this kind of lighting.
Here is a very gloomy place:
![](https://i.stack.imgur.com/16pgz.jpg)
People often extend the primary meaning to describe anything related to that emotion, even if doesn't necessarily involve dim light, though usually the concept of dim light "colors" the description. For example, the poet Langston Hughes wrote these words to describe the closing of the Newport Jazz Festival in 1960:
It’s a gloomy day at Newport. It's a gloomy, gloomy day. The music’s going away. (Source.)
Notice that the noun modified by "gloomy" is "day"; and the day is called gloomy to express the sad and discouraged emotion resulting from the loss of the festival.
The adjective "grim" describes a mental attitude of determination to do a cruel or harsh deed, usually serving a very serious purpose, such as executing a convicted murderer. Secondarily, it suggests the facial expression of a person who has resolved to carry out such a deed and will not be stopped from doing it. For example, death is often personified as "the grim reaper", usually depicted like this:
![](https://i.stack.imgur.com/mmdtT.jpg)
So, when call a place gloomy, you suggest dim lighting and/or a melancholy mood. When you call a place grim, you suggest that it's used for grave purposes and/or gruesome deeds. The sorts of places you would most easily call grim are gallows, execution chambers, slaughterhouses, prisons, battlefields (especially just after the battle), mortuaries, morgues. People often imagine that grim places are also gloomy; for example, in a painting or movie, you're likely to see a corpse-strewn battlefield depicted under a dark sky. So, there's naturally some overlap in how people use and understand these words.
Source for the gloomy image.
Source for the image of the grim reaper.
"Mandatory" means that the thing must be done due to some reason or rule.
These courses are mandatory.
That is, you must take the courses or else you fail.
Wearing helmets was made mandatory a few years ago.
It is a rule that you must wear helmets.
"Indispensable" is that the thing is so critical, useful, or important that you cannot conceivably throw the thing away.
The volunteers' help was indispensible.
The mission would have failed if the volunteers didn't help.
He made himself indispensible to the parish priest.
He was deemed a must-have person because of some good quality.
These great tools are so versatile that they are indispensable.
The tools are so good that you shouldn't be without them.
To sum up, these two words are not interchangeable - they have their own distinct meanings.
Best Answer
They are all meant to mean different classes or sizes of goods vehicle. However, there are differences between US and British English. As I am British, I will use the definitions in BrEng as a starting point and note the US variations that I'm aware of. I welcome any comments to help me improve the definitions for US English.
A van is the smallest - the sort of vehicle a sole tradesman, such as a carpenter, might drive. There are different sizes of van, but they usually have just 4 wheels and don't require any special licence to drive them because they are not so much bigger than a car.
In British English, a truck is the middle class of these vehicles. An example of these might be the kind of vehicle used by delivery services like UPS, or what you would hire if you were moving home. They are large, but not so large that they couldn't travel on a suburban street. Again, most of these can be driven on a British car driver's licence. I note that in US English they call refuse collection vehicles "garbage trucks", which are a similar size and class.
A "lorry" is the largest, and may also be a called an articulated lorry or a heavy goods vehicle (HGV). These normally only travel on major roads and carry the largest quantities. In England, you also have to hold a special licence to drive them, making them a different class of vehicle. I note that in US English these seem to be called "trucks" and those who drive them "truckers".
To sum up - "van" and "lorry" are clearly different and refer to the smallest and largest vehicles of these classes. "Truck" seems to be a more generic word to describe the harder-to-define middle-size vehicles. Some of those middle-sized vehicles may get referred to as "vans" if they are not particularly large; likewise, some lorries get called "trucks".