When should I use "is" and when should I use "are"?

I always mixed them up, and use the wrong one on other sites on StackExchange. And the questions is edited by another person.

When should I use "is" and when should I use "are"?

Sometimes I write "Is there any ..." and sometimes I write "Are there any", and I guess that one of them is wrong. Is there any good rule of thumb for this?

Sometimes I write "Is there any ..." and sometimes I write "Are there any", and I guess that one of them is wrong. Is there any good rule of thumb for this?

Yes, the rule of thumb is "use 'is' for singular and 'are' for plural".

Singular:

Is there any caffeine in decaffeinated coffee?

Plural:

Are there any words that rhyme with orange?

Note that the reason why the first sentence is in the singular form is that caffeine is a non-countable noun. You wouldn't ask "how many caffeines are there...?", but "how much caffeine is there...?"

A common cause of confusion - even for native speakers of English - arises when using a collective noun in phrases such as "a group of people" or "a team of engineers".

In such cases, the verb should agree with the collective noun. "A group" is singular even though it contains many people.