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.

---
- hosts: all
  tasks:
  - ansible.builtin.debug: 
      msg: the foo variable does not equal 'bar'
    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.

---
- hosts: all
  tasks:
  - ansible.builtin.debug: 
      msg: the foo variable does not equal the bar variable
    when: foo != bar
...

Or you may want to pipe a variable to a boolean, integer, or string. In this example, the bar variable is piped to a string.

---
- hosts: all
  tasks:
  - ansible.builtin.debug:
      msg: the foo variable does not equal the bar variable piped to a string
    when: foo != bar|string
...

AVOID TROUBLE

Jinja2 double curly braces cannot be used in the when parameter

---
- hosts: all
  tasks:
  - ansible.builtin.debug:
      msg: this will cause the playbook to fail fatal because Jinja double curley braces cannot be used in a when statement
    when: foo != {{ bar }}
...

Here is how you can use variables in a when statement. In this example, the variable named item is used in the when statement.

In this example, target_directory will be set to /tmp/foo if my_directory is /tmp/foo or target_directory will be set to /tmp/bar if my_directory is /tmp/bar.

---
- hosts: all
  tasks:  
  - ansible.builtin.set_fact:
      target_directory: /tmp/{{ foo }}
    when: my_directory is regex '/tmp/'+item
    with_items:
    - 'foo'
    - 'bar'
...

Multiple when statements can be used.

---
- hosts: all
  tasks:
  - ansible.builtin.debug: 
      msg: multiple when statements
    when: foo != hello and foo != world
...

Or like this, using or.

---
- hosts: all
  tasks:
  - ansible.builtin.debug:
      msg: multiple when statements using or
    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.

---
- hosts: all
  tasks:
  - ansible.builtin.debug:
      msg: multiple when statements
    when:
    - foo != hello
    - foo != world
...

Or like this, using or.

---
- hosts: all
  tasks:
  - ansible.builtin.debug: 
      msg: multiple when statements using or
    when:
    - foo != hello or
    - foo != world
...

Here is an example of a when statement that has multiple and & or conditions.

---
- hosts: all
  tasks:
  - ansible.builtin.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 jinja if else statement.

---
- hosts: all
  tasks:
  - ansible.builtin.set_fact:
    bar: "{%- 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.

---
- hosts: all
  tasks:
  - ansible.builtin.set_fact:
    bar: "{%- if 'hello' in foo -%}Hello
          {%- elif 'world' in foo -%}World
          {%- else -%}false
          {%- endif -%}"
...

Or like this.

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

Here are more common when statements.