Wenn

WENN has three distinct meanings.

1.)Wenn means if in the conditional sense.

                        Wenn wir in Wien sind, besuchen wir Ute.
                         ==> If we are in Vienna, we'll visit Ute.

2.) Wenn can mean whenever, in the sense of repeated events, in both the past and the present.

                        Wenn wir nach Wien fuhren, haben wir Ute besucht.
                        ==> Whenever we drove to Vienna, we visited Ute.

                        Wenn wir in Wien sind, besuchen wir Ute.
                        ==>Whenever we are in Vienna, we visit Ute.
 
3.)  Wenn can describe an event in the future.

                        Wenn wir nach Wien reisen, besuchen wir Ute.
                         ==> When we travel to Vienna, we'll visit Ute.
 
 
 

Wann

Wann is always a question word, used in both direct and indirect questions.

                        Wann ist das passiert?
                         ==>  When did that happen?

                        Ich weiß nicht, wann das passiert ist.
                         ==>  I don't know when that happened.
 
 
 

Als

Als means when, as refers to an event or state in the past and requires the simple past tense

                        Als wir in Wien waren, haben wir Ute besucht.
                        ==> When we were in Vienna, we visited Ute.
 

The conjunction "als" is commonly used to express that two events took place at the same time.  The ALS-clause establishes a point of reference in the past for an action or event described in the main clause.

                        Meine Familie hat einen Hund gekauft, als ich acht Jahre alt war.

                         ==>My family bought a dog when I was eight years old.
 

ALS introduces a dependent clause.  Thus the verb is found at the end of the clause.

The ALS-clause may also introduce a sentence.

                         Als ich acht Jahre alt war, hat meine Familie einen Hund gekauft.