appropriate, proper, relevant

These words sound similar, but are not clear for me to use.
How can I use these words in the sentences?
(Do I need "the" here???)
Could you please tell me a difference?
I'll appreciate it very much if you give me some examples, too.
I'm not great on the "rules" but here's some appropriate usage:

This is the appropriate/proper way to use it.
That's the proper/appropriate way to behave.
Your post is completely relevant in this forum.

I hope these examples are relevant and I've understood your question properly.
Yes, the two words have almost exactly the same meaning.
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According to your examples, can we use "appropriate" and "proper" in the same way?
 Mike in Japan's reply was promoted to an answer.
Students: We have free audio pronunciation exercises.
How about the word "relevant"?
Relevant; relating to the matter in hand.
As Hitch said; "Your post is completely relevant in this forum."
Your post is related to this forum. It is not related to something else.
Opposite is irrelevant. For example, 'Frankenstein's comment was irrelevant. It was not related to what we were discussing.' We were talking about tennis, but he made a comment about cooking. His comment was irrelevant.