Foreshadowing.

Firstly,
What is foreshadowing?

It’s a trail of bread crumbs and hidden clues. It’s ominous weather and shadowy figures. It’s offhand comments that only make sense in context. It’s hints of mystery, loose ends leading to something deeper…

fore·shad·ow /fôrˈSHadō/
verb
be a warning or indication of (a future event). (definition from the Oxford Dictionary)

When you foreshadow, you give your readers tiny details that look largely innocent but don’t quite fit into place. You give tiny tidbits that tease at what is to come.

Foreshadowing is most easily found in genres like mystery and thriller. However, it can be used anywhere — as long as there’s a thing that some people (you as the writer, knowledgeable characters) know and that others (the reader, main characters) don’t. Actually, that’s why thriller and mystery have the most foreshadowing — because there’s a lot we don’t know and a lot we need to find out.
But since there are secrets in every genre, foreshadowing is always useful!

When should I use foreshadowing?

As often as you can — but especially when a secret is going to come out! Also, if you ever want a hint of intrigue or mystery, foreshadowing is there for you.

And, finally…,

How do I foreshadow?

Well, one way is to subtly change the way your characters act. For example, if a character is lying, they might act a bit strange — flinching, or looking away, or just acting a little bit suspicious. Or if a character is secretly a vampire, they might conveniently disappear if they ever have to go outside (e.g. They walked outside. “Where’s *vampire name*?” someone asked. “I swear they were just here!”).

Another type of foreshadowing that’s… less subtle is what I’ll call atmospheric foreshadowing. These are changes in the background — for example, the sky turning darker or a phone buzzing for no reason could foreshadow that something bad is about to happen.
Don’t overuse this, though, because that can have the opposite effect — it spoils what’s about to happen, and deflates the suspense instead of increasing it.

So yeah!
That’s about it for foreshadowing. Foreshadowing is a great tool to add mystery and intrigue to any writing. Just remember that if you give your readers too many clues, they might just figure out the plot all the way

Great! Thanks, this helped a lot