How to express myself in the suitable form?

[phoenixtree]

As we know that there are a lot of forms which express the same idea in English.
How should I know which form to chose under a certain circumstance?
For instance, some expressions are used in formal occasions, but some are not.
My difficult is that how I can separate some of them from the others.
Thanks!

[Casiopea]

Originally Posted by phoenixtree

As we know that there are a lot of forms which express the same idea in English.
How should I know which form to chose under a certain circumstance?
For instance, some expressions are used in formal occasions, but some are not.
My difficult is that how I can separate some of them from the others.
Thanks!

Would you have a few examples?