What are subordinating conjunctions?
A conjunction is a word, or words, used to connect two clauses together. Words such as: 'although', 'because' or 'when' .
A subordinating clause is a part of a sentence that adds additional information to the main clause. A subordinating conjunction is simply the word/words that is used to join a subordinating clause to another clause or sentence.
"He was annoyed, the train had stopped."
"He was annoyed because the train had stopped."
By adding 'because' we are linking the subordinating clause "the train had stopped" with the main clause "He was annoyed".