Conjunctions serve to bind phrases and clauses together and describe the relationship(s) between them. In German there are two types of conjunctions:
Coordinating conjunctions coordinate two equal phrases: bread and butter, white wine or red wine. Examples of coordinating conjunctions include: und, oder, aber, and denn.
Subordinating conjunctions subordinate one phrase or clause to another. This often indicates a logical or causal relationship, as in I stayed home because I was sick. The two clauses are not equal: I didn't just stay home, and I wasn't just sick, but the reason that I stayed home was because I was sick. Examples of subordinating conjunctions include weil, als, wenn, wann, ob, and nachdem.
Conjunctions are words that link two or more words, clauses, phrases or sentences. In German, they belong to the group of non-declinable words. However, in German you will often find several posibilities to choose from to denote one single English conjunction. Such is the case with aber and sondern.
Most of the interesting conjunctions in German are subordinating. Subordinating conjunctions induce the verb at the end behavior; with coordinating conjunctions the verb in each clause is in 2nd position.
Das Kind will nicht nach Hause gehen, sondern in den Park.
The child doesn't want to go home, but to the park.
Sie ist erschöpft, aber sie will nicht schlafen gehen.
She is exhausted, but she doesn't want to go to sleep.
As you can see, both aber and sondern mean
but in English. How do you know which but conjunction to use? It is actually quite simple: aber, which means but, is used after either a positive or negative clause. On the other hand, sondern is only used after a negative clause when expressing a contradiction. In other words, the first clause of the sentence must contain either nicht or kein, and the second part of the sentence must contradict the first part of the sentence. Sondern can be translated as but, on the contrary, instead, but rather.
Coordinating conjunctions connect sentences, clauses, phrases, or words of equal rank. These coordinating conjuctions are and, but, and or.
In German, the most frequently used coordinating conjunctions are:
Keep in mind that the word order does not change after coordinating conjunctions. You must place the subject first and the verb second, following the normal word order in a sentence.
All of the subordinating conjunctions listed below require the conjugated verb to go at the end of the clause they introduce. An easy way to learn them is to learn the ones that are not subordinating, since there are fewer of those.
Some of the subordinating conjunctions can be confused with their second identity as prepositions (bis, seit, während), but this is usually not a big problem. The word als is also used in comparisons (größer als, bigger than), in which case it is not a subordinating conjunction. As usual, you have to look at the context in which a word appears in a sentence.
All interrogative words (wann, wer, wie, wo, etc.) can also be used as subordinating conjunctions.