sth. = SOMETHING

Something means a thing that is unknown. It is often used in positive sentences.

Anything means a thing of any kind. Use it in questions and negative sentences.
It can also be used to to mean I don't mind.

Nothing means no single thing. Used as subjects of a sentence or question.

Everything means all the things. Use in affirmative, negative and interrogative sentences
when we want to be inclusive.


source: 
ecenglish.com

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Exceptions

Positive Clauses with Any

We usually use some in positive clauses.
But after never, without, hardly, we use any.
Example:
We never go anywhere.
She did her homework without any help.
There’s hardly anyone here.
Also in if clauses, we usually use any.
Example:
If there is anything to do, just call me.

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exercises online:

ecenglish.com

english-test.us/quiz

ego4u.com

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