Suggestions, advice and warnings

What's the difference between suggestions, advice and warnings?

A suggestion is an idea or plan put forward by someone:
I suggest you study hard for the FCE exam.

A piece of advice is someone's opinion or recommendation:
If I were in your shoes, I’d study hard for the FCE exam.

A warning is a piece of strong advice with a mention of what may happen if this advice is not followed: Don’t forget to practice writing letters of application otherwise you’ll fail your FCE writing paper.

Making suggestions
Some common structures used for making suggestions are:
Why don't you + infinitive ...?
You/We could + infinitive ...
Let's + infinitive ...
What/How about + gerund ...?
I suggest you/we + infinitive ...

Giving advice
Here are some ways of expressing advice:
I don't think you should + infinitive ...
You ought to + infinitive ...
You ought not to + infinitive ...
You shouldn't + infinitive ...
If I were you, I'd + infinitive ...
If I were in your position/shoes, I'd + infinitive ...
You'd better + infinitive ...
You'd better not + infinitive ...
Whatever you do, don't + infinitive ...

Giving warnings
Here are some expressions used for giving warnings:
Don't + infinitive ..., or you might/could + infinitive ...
Look out! Be careful!

Otherwise can be used instead of or to warn what will happen if advice is not followed: Work hard otherwise you'll fail your exam.