The first dependent clause form is the subordinate clause, which is formed when a a subordinating element, such as a subordinator, introduces a clause by immediately preceding the subject of the clause. The subordinator, therefore, is KEY MARKER for identifying a subordinate clause. The subordinate clause is adverbial, and is overtly marked as a subordinate structure and assumes a 'part-to-whole' relationship with an independent clause. As with most adverbials, a key test for a subordinate clause is if it can move around in the sentence or be deleted, and the sentence still makes sense.

When I have a free hour, I'll finish my homework.

I'll finish my homework when I have a free hour.

I'LL FINISH MY HOMEWORK.

He waited while I cashed a check.

While I cashed a check, he waited.

HE WAITED.

Obviously, the subordinate clause adds important information, but it is not required information for the independent clause to be understood as written.

Don't be confused by prepositional phrases and subordinate clauses. While prepositions and subordinators may appear similar (and are often drawn from the same list of words), you should be able to distinguish between a prepositional phrase (preposition + nominal object) and a subordinate clause (subordinator + clause).

At midnight, I went to the market.

When the clock struck midnight, I went to the market.

While both the prepositional phrase and the subordinate clause are functioning adverbially (you can move them around in the sentence: I went to the market at midnight; I went to the market when the clock struck midnight), you should be able to recognize the differences between the highlighted structures. 

To test your understanding of the concepts discussed on this page, begin with the link below for an example practice exercise:

SUBORDINATE CLAUSE PRACTICE EXERCISE

For a bit more of a challenge, analyze the following passage for subordinate clauses.

Margot’s mind reeled. Since she could hear the listless, scraping sounds of fingertips against the door, it haunted her to consider the sea of faces outside. They were faces she once knew in a different light, but now they were putrid reflections of squalor and decay. There was nothing left for her to glean from their lifeless eyes except the grim reminder that she was alone. She tried to ignore what she thought she saw, even though she was the last bastion of humanity. Death awaited her outside.

To review your answers to these two samples, check the SUBORDINATE CLAUSE SAMPLES ANALYSES page.