Conditionals

The if, else, and unless statements come from Ruby.

if statement

{% if expression %}
    {{ value }}
{% endif %}

Example using Google Analytics

 {%- if jekyll.environment == 'production' and site.google_analytics -%}
     {%- include google-analytics.html -%}
 {%- endif -%}

else statement

{% if expression %}
    {{ value }}
{% else %}
    {{ other_value }}
{% endif %}

Example, using assignment:

{% if expression %}
    {% assign answer = "yes" %}
{% else %}
    {% assign answer = "yes" %}
{% endif %}

{{ answer }}

unless statement

{% unless expression %}
    {{ value }}
{% endunless %}

This is a negated if statement.

So you could have equally done:

{% if not expression %}

Using unless can be useful for adding a separator between items.

<p>
    {% for item in page.my_array %}
        {{ item }}
        {% unless forloop.last %}, {% endunless %}
    {% endfor %}
</p>

Consider also using the join filter which will do something similar for joining strings as one string.