All three words mean to do something--usually to speak--in a way that is designed to be impressive, and they all carry the connotation that the thing added is not of real value; that it's just for show. The difference is in what is added.
With bombast the speaker adds something big and showy. Think of a popular preacher; loud, self-satisfied, and full of his own importance. This is more likely to be used when the speaker is puffing up his or her religion, or politics, or patriotism.
With grandiloquent the speaker adds something elaborate or complicated. Think of a diplomat standing in front of a bored audience giving a long speech full of complicated metaphors. This is usually going to be a highly formal speech, often too formal for the occasion (or for any occasion).
Bombastic and grandiloquent are close enough in meaning that they can be used almost interchangeably. There is, however, a difference in implication. A bombastic speech is more likely to be loud and aggressive; a grandiloquent speech to be formal and academic. But like I say, this is not a hard and fast rule, just a mild implication.
With ostentatious the speaker is showing off. This is similar to "bombast," but "ostentatious" is more often used for actions rather than speech. For example, if you drive your Ferrari up to the restaurant, walk past the valet and into the restaurant, and hand the waiter your keys, saying loudly, "Have someone take good care of my very expensive Ferrari!" you are being ostenatious. Most often, this is used to describe someone who is showing off their money.
A good example of a bombastic, grandiloquent speech is the Reverend Chadband in Dickens' Bleak House. Here is is first speech, on arriving at a family dinner.
“My friends,” says Mr Chadband, “peace be on this house! On the master thereof, on the mistress thereof, on the young maidens, and on the young men! My friends, why do I wish for peace? What is peace? Is it war? No. Is it strife? No. Is it lovely, and gentle, and beautiful, and pleasant, and serene, and joyful? Oh, yes! Therefore, my friends, I wish for peace, upon you and upon yours.”
This is a good example of "grandiloquent." It's overly flowery, overly elaborate, for the occasion, saying something in a hundred words that could easily be said in five.
Here is a later speech by him to the same dinner-party that would more likely be described as bombastic:
"It is," says Chadband, "the ray of rays, the sun of suns, the moon of moons, the star of stars. It is the light of Terewth."
Mr. Chadband draws himself up again and looks triumphantly at Mr. Snagsby as if he would be glad to know how he feels after that.
"Of Terewth," says Mr. Chadband, hitting him again. "Say not to me that it is NOT the lamp of lamps. I say to you it is. I say to you, a million of times over, it is. It is! I say to you that I will proclaim it to you, whether you like it or not; nay, that the less you like it, the more I will proclaim it to you. With a speaking-trumpet! I say to you that if you rear yourself against it, you shall fall, you shall be bruised, you shall be battered, you shall be flawed, you shall be smashed."
This is still grandiloquent, still over-elaborate, but it also has that bullying, over-the-top quality that is often associated with "bombast."
The phrases "in the afternoon" and "in the evening" are similarly to the phrase "in the morning." It is possible to use them to refer to tomorrow afternoon, tomorrow evening, and tomorrow morning respectively. However, these phrases are dependent on context. Using them in the wrong way or at the wrong time will cause confusion -- or at least sound odd.
I go into detail in the sections below. Go to the bottom for a summary.
How to Use These Phrases
"In the morning," "in the afternoon," and "in the evening" are all used to describe approximate time frames. Though the exact day may be stated, the exact time is either unknown or unimportant.
There are several different ways to use these phrases as illustrated below.
Reoccurring or Routine Events
You can use the simple present tense with any of these phrases to describe events or situations that occur with some frequency.
For example:
I drink coffee in the morning, but I prefer tea in the afternoon. I am rarely thirsty in the evening.
Here the speaker is describing his (or her) habits. Each morning, he will drink coffee. Each afternoon, if he drinks something, it will likely be tea. During the evening, though, he is unlikely to drink anything since he rarely thirsty at that point in the day.
Statements like this tend to include a description of the frequency such as "always", "rarely", or "sometimes."
Past Events
You can use the past tense to describe events that happened during an approximate time frame on a specific day.
As an example, let's say a woman named Mary did the following things on December 25, 2018 (Christmas Day):
- 8:17 AM to 10:49 AM: Opened presents with her husband and children
- 1:21 PM to 5:05 PM: She and her family visited her parents
- 6:33 PM to 9:46 PM: She and her family visited her in-laws
To describe her day, Mary might say the following:
I was very busy last Christmas! My family opened gifts in the morning. We then visited my parents in the afternoon and my in-laws in the evening.
When describing the past, it's important to establish which day you are talking about. In my experience, no one ever uses "in the morning/afternoon/evening" unless a specific day has already been mentioned.
Future Events
You can use the future tense to describe events that will (or may) happen in the future. It is often used to describe someone's plans.
For example, let's say a man named John is planning his weekend. His itinerary for Saturday might be as follows:
10:00 AM to 12:00 PM -- Go fishing
2:00 PM to 5:00 PM -- Play golf
7:00 PM to 11:00 PM -- Hang out with friends at the bar
John might summarize his plans this way:
I'm going to have a nice, relaxing day on Saturday. I'm going fishing in the morning, playing golf in the afternoon, and hanging out with my friends at the bar in the evening.
When describing the future, specifying a day is optional. If a day has not been specified, the assumption is that you are referring to the next morning, afternoon, or evening. Depending on the time of day, that may mean today or tomorrow.
Why the Confusion?
Why is "I've got to go to work in the morning" acceptable but not "I've got to go to work in the afternoon" and "I've got to go to work in the evening"? All three phrases are grammatically correct, the later two are likely to be inappropriate for a number of reasons.
(By the way, you can shorten the phrase to "I've got work in the morning.")
First of all, English speakers tend to start work in the morning (i.e. prior to noon). Someone wouldn't say they are "going to work in the afternoon" (or evening) unless they had a shift that started late. However, anyone who starts work in the morning and leaves in the evening might say they "have work in the afternoon."
Secondly, the line between morning and afternoon is clearer than the line between afternoon and evening. "Afternoon" literally means "after noon" so it's clear that "in the morning" is referring to any time prior to 12:00 AM. Evening doesn't have as precise a starting point, so it's less clear when something is "in the afternoon" or "in the evening." For this reason, "in the afternoon" and "in the evening" aren't used as often.
Lastly, and most importantly, the meaning of these phrases can change depending upon the time of day. If it's 8:00 PM when you say "I've got work in the morning," it's clear you mean tomorrow morning. If you to say the same thing at 8:00 AM, it would be a little confusing, but most people would assume you meant "tomorrow morning" and not "this morning." However, if you were to say "I've got work in the afternoon" at 8:00 AM, people would assume you meant "this afternoon" and not "tomorrow afternoon" since "this afternoon" is the next time it will be the afternoon.
Examples
To clarify this point more, consider the following example. Bob and Alice are co-workers. Bob has come to Alice asking for help. Alice responds by telling him when she's available. Here are three ways the conversation might be worded (after Bob and Alice greet each other.)
Example #1
Bob: Can you help me with this task tomorrow?
Alice: Yes. I have meetings in the morning, but I am free in the afternoon.
Example #2
Bob: Can you help me with this task?
Alice: Yes. I have meetings tomorrow morning, but I am free in the afternoon.
Example #3
Bob: Can you help me with this task?
Alice: Yes. I have meetings in the morning, but I am free in the afternoon.
In example #1, when Bob asks for help, he mentions tomorrow. This establishes "tomorrow" as the context for both "in the morning" and "in the afternoon." This conversation could take place at any time during the day because Bob makes it clear that he wants help tomorrow.
In example #2, when Alice responds, she mentions tomorrow by saying "tomorrow morning." This establishes "tomorrow" as the context for "in the afternoon." This conversation could also take place at any time during the day and be clear. However, Alice's response would only make sense if the conversation happened near the end of the work day (e.g. 4:00 PM.) In this case, Bob would assume Alice is busy for the remainder of the work day. If Alice responded the same way earlier in the day (e.g. 8:00 AM), she is omitting her availability for that day. There's a number of ways Bob might react. He might ask "Are you free today?", he might assume she is busy until tomorrow, or he might suspect she is trying to postpone helping him.
In example #3, neither mention tomorrow, today, or any other day. If the conversation happened at 8:00 AM, Bob would assume Alice meant "this morning" and "this afternoon." If the same conversation happened at 4:00 PM, Bob would assume Alice meant "tomorrow morning" and "tomorrow afternoon."
Summary
In short, "in the morning/afternoon/evening" can be used to describe:
- Something that happens on a reoccurring basis
- Something that happened on a specific day in the past
- Something that will happen on a specific day in the future
- Something that will happen the next time it is morning/afternoon/evening
I hope the explanation above clarifies how to use these phrases. They are a good example of how important context can be when speaking English.
When in doubt, you can say "this morning/afternoon/evening" or "tomorrow morning/afternoon/evening" instead.
If someone else uses one of these phrases and you aren't sure what day they are referring to, don't hesitate to ask! (Most native English speakers I know would prefer to clarify than be misunderstood.)
Best Answer
They're not quite synonymous, although in this context some of them are interchangeable.