The when parameter (which is like an if statement) can be used to do something when a condition evaluates to true or false.

when: <some condition>

Often, the when parameter is used to compare a variable to a boolean (such as true, false, 1, or 0), to a string (such as Hello World), or to an integer (such as 12345). In this example, the when parameter is used to determine if the foo variable contains string "bar". Notice foo is not wrapped in quotes and bar is wrapped in quotes. Since foo is not wrapped in quotes, foo is interpreted as a variable. Because bar is wrapped in quotes, bar is interpreted as a string.

- debug: 
    msg: the foo variable does not equal the bar string
  when: foo != 'bar'

The when parameter can also be used to compare a variable to another variable. In this example, the foo variable is compared to the bar variable. Notice both foo and bar are not wrapped in quotes, meaning that both will be interpreted as variables.

- debug: 
    msg: the foo variable does not equal the bar variable
  when: foo != bar

Or, you may want to convert a variable to a boolean, integer, or string. In this example, the bar variable is converted into a string.

- debug: 
    msg: the foo variable does not equal the bar variable converted into a string
  when: foo != bar|string

AVOID TROUBLE

Jinja2 statements cannot be used in the when parameter

- debug: 
    msg: this will return a warning because variables must NOT be wrapped in curley braces in a when statement
  when: foo != {{ item }}

Multiple when statements can be used like this.

- debug: 
    msg: multiple when statements
  when: foo != hello and foo != world

Or like this, using or.

- debug: 
    msg: multiple when statements
  when: foo != hello or foo != world

Or like this. This would perform an "and" situation, where all of the when statements would have to evaluate to true, not an "or" situation.

- debug: 
    msg: multiple when statements
  when:
    - foo != hello
    - foo != world

Or like this, using or.

- debug: 
    msg: multiple when statements
  when:
    - foo != hello or
    - foo != world

Here is an example of a complex when parameter that has multiple and and or conditions.

- debug: 
    msg: multiple when statements
  when: ((foo != hello) and (foo != world)) or ((bar != hello) and (bar != world))

Sometimes, for more complex situation, or to avoid multiple tasks, I will use a jinja2 if else statement.

- set_fact:
  foo: "{%- if foo == 'hello' -%}Hello
        {%- elif foo == 'world' -%}World
        {%- else -%}false
        {%- endif -%}"

And here is a jinja2 if else statement that checks if the foo variable contains hello or world.

- set_fact:
  foo: "{%- if 'hello' in foo -%}Hello
        {%- elif 'world' in foo -%}World
        {%- else -%}false
        {%- endif -%}"

Or like this.

- set_fact:
  foo: "{%- if foo | regex_search('.*hello.*') -%}Hello
        {%- elif 'world' in foo -%}World
        {%- else -%}false
        {%- endif -%}"

This table lists common when statements.