Is it possible to use wenn when referring to the past? I've researched that wenn is used exclusively for the present and future, while als is used for the past.
For example, I was trying to figure out how to say
When I had a lot of homework to do, I was irritated.
Wenn ich viele Hausaufgaben habe, mache ich geärgert.
How would I say that in proper German?
Wenn ich viele Hausaufgaben hatte, war ich immer ärgerlich.
The rule you quote is not exactly right. What matters is the habitual aspect, not whether the described events are in the past or in the future. As a rule of thumb, whenever you use 'whenever' (rather than just 'when') in English, you can use 'wenn' or 'immer wenn' in German.
(A difference exists for the conditional or temporal sense: future situations prefer 'wenn' and past situations prefer 'als', but your example is definitely a case of habitual aspect.)
The original and your translation don't match here, starting with my impression that the English sentence When I had a lot of homework to do, I was irritated is not clear. That is because it is in the past and lacking a context. So, two kinds of translations seem possible:
Es störte mich (immer), wenn ich viele Hausaufgaben zu erledigen hatte. (conditional clause)
als auch:
Als ich (einmal) viele Hausaufgaben zu erledigen hatte, ärgerte mich das. (temporal clause)
However, the first version would rather assume a phrasing like "I used to be irritated when I had a lot of homework to do".
As soon as you put the English original into present tense, it becomes clear what should be the proper translation:
Wenn ich viele Hausaufgaben habe, stört mich das.
Thus, keep in mind as a rule of thumb:
When (conditional) = Wenn
When (temporal) = Als
If you are uncertain, this rule of thumb may work:
"Als" can be used when referring to "that one time" a more or less precise time, for example: "Als ich ein Kind war"
Wenn can be used in the other cases more or less. Be careful that for indirect questions "wann" should be used instead.
