Phrases and clauses are both groups of words that work together in a sentence. The difference is that a clause has a subject and a verb—often, a clause could be a sentence if it were all by itself, and when it could be, we call it a main clause or an independent clause.
Zombies hunt the surviving humans. [main clause]
A phrase, on the other hand, is missing something. Phrases work within sentences. There are prepositional phrases, noun phrases, and so on. Phrases can play a lot of different roles in a sentence, but they work with main clauses. Somewhere, every sentence must have a main clause. Here’s an example of a prepositional phrase followed by a main clause.
On the TV show The Walking Dead, zombies hunt the surviving humans.
[prepositional phrase] [main clause]
Subordinate clauses get their name from the fact that they always start with a subordinating conjunction such as since, because, although, and while.
“Subordinate clause” is the name you’re more likely to have learned in school, but the other name, “dependent clause,” may help you better understand what they do because adding that subordinating conjunction to the head of the clause makes it dependent on a main clause. A dependent clause without a main clause is a fragment. The dependent clause needs the main clause—it depends on the main clause—to make it a proper sentence. Fragments are generally frowned upon in business writing, but you do often see them in fiction, especially in dialog because they create a conversational, punchy, informal tone.
I know summer is right around the corner. [main clause]
The days are getting longer. [main clause]
Because the days are getting longer. [fragment]
I know summer is right around the corner because the days are getting longer. [complex sentence with a main clause followed by a dependent clause]
You may have been taught not to start a sentence with the word because (one of the subordinating conjunctions), but that is a fib used by beleaguered teachers to keep small children from writing sentence fragments (e.g., Before Bobby leaves! Unless Marta brings cookies! When the ice cream man comes!)
A subordinate clause can go at the beginning of a sentence or later in a sentence. The only difference is that if it goes at the beginning, you need a comma after the subordinate clause, and if goes later, you don’t need a comma.
Here are some examples with the subordinate clause at the beginning of the sentence: