In German there are independent clauses, or main clauses, and dependent clauses or subordinate clauses. An independent clause is a clause or sentence that can stand alone as a complete sentence. It typically follows regular word order (Subject-Verb-Object). We can connect two or more main clauses together, using coordinating conjunctions. (e.g. "und:, "aber", etc. The word order in each main clause is unchanged.
Ich gehe heute ins Kino. Er geht heute in die Arbeit. (two independent clauses)
I go to the movies. He goes to work.
Ich gehe heute ins Kino und er geht heute in die Arbeit.
I go to the movies and he goes to work.
In the above example, we combined or connected to independent sentences to form a longer sentence, using the coordinating conjunction "und" (and).
1. Subordinate Clauses
A sentence that cannot stand alone is called a dependent clause. It needs to be connected to a main/independent clause in order to make sense. We connect a subordinate or dependent clause to a main clause by using a subordinating conjunction. In English, this does not change the word order. In German, however, it does. Subordinating conjunctions change regular word order to subordinate or dependent word order. In other words, the conjugated verb is moved or "kicked" out of second position and placed at the end of the clause.
Sie bleibt heute zu Hause. Sie ist sehr müde. (two independent clauses)
She is staying home today. She is very tired.
Sie bliebt heute zu Hause, denn sie ist sehr müde. (two ind.clauses and coord. conj.)
Sie is staying home today as she is very tired.
Sie bliebt heute zu Hause, weil sie sehr müde ist. (ind. and dep. clause and subor. conj.)
Notice that the word order did not change when using a coordinating conjunction, but the verb was kicked to the end of the sentence in the subordinate clause, due to the subordinating conjunction. Both conjunctions mean ‘because,’ but only ‘weil’ forces the verb to the end of the clause. Also, be aware that a subordinate clause can be extended by way of a coordinating conjunction, but this does not change the fact that the verb is placed at the end of the respective clauses (the subordinate word order continues throughout the combined dependent clauses).
2. Subordínate Clauses at the beginning of a sentence
It is also possible to start a sentence with a subordinate clause. This can be done to put emphasis on this part of the sentence. Starting a sentence with the subordinate clause creates inverted word order in the main clause. One could say that the entire subordinate clause is now in position one
Sie bliebt heute zu Hause, weil sie müde ist. (main clause, subordinate clause)
She is staying home today because she is tired.
Weil sie müde ist, bleibt sie heute zu Hause. (subordinate clause, main clause)
Because she is tired, she is staying home today.
Notice that in the top example, the main clause follows regular word order, and in the subordinate clause the verb was kicked to the end. In the bottom example, the subordinate clause is at the beginning of the sentence with the verb kicked to the end of the clause. The main clause now is in inverted word order.