Stefan Reisich is correct for a simple condition but it won't work if it is little complex condition like in for loop it would fail
eg:

<span t-foreach="l.tax_id" t-as="t">
       <span t-if="t.domain=='ipi'"><t t-esc="t.amount" /></span>
       <span t-if="not t.domain=='ipi'"><t t-esc="0.00" /></span>
</span>

The above code will print "0.00" as many times as first condition fails but we would want to print that single time. So I did a workaround using a variable and set its value and print value outside for loop. eg:

<t t-set= "amount" t-value="0.00" ></t> 
<span t-foreach="l.tax_id" t-as="t">
     <span t-if="t.domain=='ipi'"><t t-set= "amount" t-value="t.amount" /></span>
</span>
<span t-esc="'%.2f'%(amount * 100)"/>

So what above code does is, it sets value of amount variable to "0.00" and checks for condition in for loop, it it is found correct, it resets value of "amount" variable and finally we use this variable outside for loop.

It works great!!

Cheers!! 

From https://www.odoo.com/documentation/8.0/howtos/web.html#the-qweb-template-engine:

The basic alternative block of QWeb is t-if:

<t t-name="HomePageTemplate">

<div>

<t t-if="true == true"> true is true </t>

<t t-if="true == false"> true is not true </t>

</div>

</t>

Although QWeb does not contains any structure for else.

\https://www.youtube.com/watch?v=b2Z0PT0GTCI&list=UUZVWw0iLx0pFTzUzvl_XpcA&index=1