Learn Italian: Non interrompere l'azione! - Do not interrupt the action!

In Italian, when you tell someone not to do something informally (the tu form), you use non followed by the infinitive of the verb. So, 'don't interrupt' translates to non interrompere. If you were telling them to do it (a positive command), you would use the conjugated form: interrompi!.

This lesson is about verbs that take di, so why isn't there a di before l'azione?

The rule for using di (or a) applies when a verb is followed by another verb (an infinitive), like in smettere di parlare (to stop talking). Here, interrompere is followed directly by a noun (l'azione). Since it takes a direct object, no preposition is needed between the verb and the noun.

How can I tell if azione is masculine or feminine since it has an l' in front?

It is feminine. A highly reliable rule in Italian is that words ending in -zione are almost always feminine. Because it starts with a vowel, the feminine article la drops its final letter and joins the word to become l'azione.