Subordinating conjunctions

There are many subordinating conjunctions, and they all share two characteristics: (1) they introduce dependent clauses that do not make complete sense when they stand alone; and (2) the conjugated verb in such clauses is normally the last element in the clause:

main clause + subordinating conjunction + dependent clause + verb
Ich besuchte Karl, + als + ich in der Hauptstadt 1 war.
I visited Karl when I was in the capital.
Using subordinating conjunctions
Some of the most commonly used subordinating conjunctions are:
als                                when                     ob                           whether, if
als ob (als wenn)      as if                        obwohl                  although
bevor                          before                    seit(dem)              since
bis until,                    by the time           sobald                    as soon as
da                               since                       sooft                       as often as
damit                         so that                    soviel                     as far as
dass                           that                          während               while
ehe                              before                    weil                        because
falls                            in case                    wenn                      when(ever), if
nachdem                   after                        wie                         as
Let’s look at a few example sentences. Take note of where the conjugated verb
stands in the dependent clause:
Ich weiß, dass du lügst. I know that you’re lying.
Er konnte nicht einkaufen gehen, He couldn’t go shopping, because he’s
weil er pleite ist. broke.
Als sie in Paris war, kaufte sie sich When she was in Paris, she bought a
ein paar neue Kleider. couple new dresses.
Notice that in the first two example sentences above, the dependent clause is the second clause. In the third example, the dependent clause is the first clause. These clauses function in the same way no matter what their position in the sentence.
Whatever tense the verb is in, the conjugated verb or auxiliary will be the last element of a clause introduced by a subordinating conjunction:
..., dass er krank ist. . . . that he’s sick.
..., dass wir nichts gehört haben. . . . that we didn’t hear anything.
..., dass ich gut singen kann. . . . that I can sing well.
There is only one exception to that rule. When a double infinitive structure is part of the sentence introduced by a subordinating conjunction, the auxiliary verb will precede the double infinitive. This occurs, of course, with modal auxiliaries and certain other verbs such as helfen, hören, lassen, and sehen (to help, to hear, to get or to have done, to see). With modal auxiliaries, for example:
..., weil sie uns wird einladen müssen . . . . , because she will have to invite us.
..., weil er mit dir hatte fahren wollen . . . . , because he had wanted to drive with you.
The same kind of word order occurs with helfen, hören, lassen, and sehen. For example:
..., weil Tina mir wird kochen helfen . . . . , because Tina will help me cook.
..., weil ich es habe reparieren lassen . . . . , because I have had it repaired.
When as a conjunction

You need to consider the conjunction when carefully. Although English uses the same conjunction for three different functions, German does not. There are three distinct German words, one for each function. 

When using when to ask a question, the German interrogative is wann:

Wann kommen die Gäste morgen?                         When are the guests coming tomorrow?
Bis wann kann ich vorbeikommen?                        Until what time can I drop by?
Seit wann wohnt Lukas in Bremen?                        How long has Lukas been living in Bremen?
The brief response to a wann-question can include wann:
Ich weiß nicht wann.                                                   I don’t know when.
In general, however, responses to a wann-question in the present tense require wenn:
Wann sind die Straßen naß?                                     When are the streets wet?
Die Straßen sind naß, wenn es regnet.                    The streets are wet when it rains.
When using when to mean whenever, the German conjunction is again wenn, a subordinating conjunction:
Wenn wir nach Bonn kommen, besuchen            When(ever) we come to Bonn, we visit our
wir unsere Tante.                                                        aunt.
Wenn Sie sich erst einmal eingearbeitet               When you’ve had a chance to get used to the
haben, werden Sie unsere Ziele besser                   job, you’ll understand our goals better.
verstehen.
Wenn es Sommer wird, schwimmen wir               When summer comes, we’ll go swimming
jeden Tag.                                                                     every day.
When using when in a past tense sentence, the German conjunction is als, also a subordinating conjunction:
Als er ankam, sah er Maria vor dem Haus                 When he arrived, he saw Maria standing in stehen.                                                                           front of the house.
Es fing an zu regnen, als wir zum Garten                   It began to rain when we wanted to go to 
gehen wollten.                                                                  the garden.
Gerade als Erik hier war, wurde meine                      Just when Erik was here,
Schwester krank.                                                              my sister got sick.
Comma usage with subordinating conjunctions

When two clauses are combined by a subordinating conjunction, the two clauses are separated by a comma:

Luise versteht, dass Benno zu Hause                             Luise understands that Benno has to
bleiben muss.                                                                       stay home.
The two forms of sentence formation with subordinating conjunctions are:
main clause + subordinating conjunction + clause with verb in final position
Er besuchte sie, + als + er in Berlin war.
He visited her when he was in Berlin.
subordinating conjunction + clause with verb in final position + main clause
Als + er in Berlin war, + besuchte er sie.
When he was in Berlin, he visited her.

Remember that the verb will precede the subject in the main clause if the sentence is introduced by a subordinating conjunction as illustrated in the previous example.