Colons are usually reserved for lists (or titles/names), either in-line or starting some bullets/numberings.

Semicolons are used mainly when either a comma (continuation) or a period (stop) could both be grammatically correct or acceptable.

it's different,

colons are commonly used for emphasis, show dialogue, introducing lists or text, and clarifying composition (the one in your example)

while semicolons from what I understand, is used to join two different sentences that stem from one same idea where using a period is too strong (as it's end a sentence and would introduce a new idea), and using a comma is too weak (as it's used to continue the previous words without ending a sentence).

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Think of it this way: Colons introduce or define something. Semicolons join two main statements.

Example: Richard had only one person on his mind: his girlfriend, Rose.

Example: I bought a new bike; the tire is flat.

I use a mnemonic based on the first letter of each word: colon = continue and semicolon = stop.

The clause after a colon will continue the idea presented in the earlier clause. The clause after the colon gives us more information in some way. In your example, the promotion is more information about “what he worked for.”

A semicolon serves as a stop. It separates two ideas that are related, just as any two sentences in a paragraph would be, but are different or contrasting in some way. As u/ikantolol wrote, it works where a period seems too strong and a comma seems too weak.

Semicolons are also used to separate items in a list when those items contain commas: “I had fun rewatching Star Wars Episode IV with my aunt, a screenwriter; her husband, an astrophysicist; and their old friend from high school, Mark Hamill.”

As is always the case in English, there are exceptions and other uses, but for me, “colon = continue / semicolon = stop” has been a good guideline.

Can colons and semicolons be used in exactly the same sentences sometimes? It may just create a different effect. For example couldn't I say:

Sandip spent three hours in the library: he couldn't find the book he wanted

Because the clause after gives more information on why he was at the library

That’s a great question. I’d need to look it up to give a more precise answer, which might or might not end up being an “ELI5” answer. But it seems like a semicolon works better in that spot because there is more of a pivot than a continuation.

I can see how we might view the second clause as a continuation: it does give more information. His inability to find the book is the reason why he spent so long in the library.

But it really seems like more of a contrasting situation. If he spent that long in the library, we would expect him to find the book that he wanted; however, he didn’t.

It seems like a semicolon works well in your example because we can so easily slide the word “however” into that sentence: “Sandip spent three hours in the library; however, he couldn’t find the book he wanted.”

If you think that a colon conveys the meaning that you’re aiming for, the word “because” probably conveys that even more clearly: “Sandip spent three hours in the library because he couldn’t find the book he wanted.”

That’s exactly the difference between a colon and a semicolon.

A colon explicitly relates the two statements. In your example, a colon would indicate that he spent three hours in the library because he couldn’t find the book. A semicolon would leave it open to interpretation: he couldn’t find the book, but that’s not necessarily why he spent three hours in the library.

So yes you can use them in the same sentence but not interchangeably. They have different meanings

thanks

Yup. Colons, semicolons, and commas have a lot of overlap. There aren’t rigid divisions between their use and a lot of situations where some or all of them are valid.

I typically think of it this way: use a colon when the second clause clarifies or adds context to the first clause; use a semicolon when the two clauses are related but one isn’t modifying the meaning of the other. To me that’s really the key: the choice depends on how you want to adjust the relationship between the two clauses. It’s a bit like putting different colored lenses over an image to adjust its mood or something.

(Note that I’m talking specifically about using colons and semicolons to join two independent clauses here. There’s lots of other uses for both marks. I also acknowledge that employing some of those other users here is potentially confusing and I apologize for that. It’s just the way I talk. I can’t help it.)

A semicolon joins two complete and related ideas.

"I need to buy some things; I'm having a party."

A colon CAN join two complete ideas. The first idea must be complete, and the second should be a list, or a definition or explanation of the first idea. Sometimes it's used to introduce a name.

"I need to buy some things: napkins, drinks, and ice."

"I'm having a party: a gathering of friends, with dancing and snacks."

"You might know one of my guests: Shakira."