Some Turkish suffixes can be added to a verb to turn a sentence into an adverbial clause, which acts as an adverb that modifies the verb in the main clause.
In this lesson learn such a suffix, namely -DİğİndE.
The meaning of this suffix is something close to when, as in when x was happening, y happened. It can refer to ongoing events as well as sequential events.
Note that there is no marker in çaldığında that conveys tense. We infer the tense from the context. Another possible translation for this sentence could be: (In general, usually) When the doorbell rings, I open it. So there is always ambiguity (like in many other suffixes that don't convey a tense), but ninety percent of the time you'll understand the sentence without problems thanks to the context.
Since this suffix helps us differentiate time into two points of reference, it is often used together with complex tensed (that is, verbs conjugated with copulae) verbs. Related to copulae you have only seen the used-to sentences so we will see examples of that. But in the next lesson, you will learn how to construct sentences using the past continuous.
Note how in the first sentence -DİğİndE referred to a future or habitual event, but in the second sentence it refers to a past event. The context determines the interpretation.
According to the subject it refers to, -DİğİndE conjugates as follows: