Some clauses in a sentence are more important than others. In English to join two thoughts that aren’t equal in importance you need to use a subordinate conjunction, but you must be careful. Subordinate conjunctions can change the meaning of a sentence dramatically.

Subordinate conjunctions emphasize that one idea (the boss, an independent clause, the equivalent of a complete sentence) is more important than the other (the employee or subordinate clause). The conjunctions joining boss and employee give some information about the relationship between the two ideas. These conjunctions are called subordinate conjunctions. Some common subordinate conjunctions are while, because, although, though, since, when, where, if, whether, before, until, than, as, as if, in order that, so that, whenever, and wherever. (Whew!)

Check out how conjunctions are used in these examples:

Michael was shaving. (not a very important activity)
The earthquake destroyed the city. (a rather important event)

If these two sentences are joined as equals with a coordinate conjunction, the writer emphasizes both events:

Michael was shaving, and the earthquake destroyed the city.

Grammatically, the sentence is legal. Morally, this statement poses a problem. Do you really think that Michael’s avoidance of five o’clock shadow is equal in importance to an earthquake that measures 7 on the Richter scale? Better to join these clauses as unequals with the help of a subordinate conjunction, making the main idea about the earthquake the boss:

While Michael was shaving, the earthquake destroyed the city.

or

The earthquake destroyed the city while Michael was shaving.

The while gives you time information, attaches the employee sentence to the boss sentence, and shows the greater importance of the earthquake. Not bad for five letters.

Here’s another:

Esther must do her homework now.
Mom is on the warpath.

In combining these two ideas, you have a few decisions to make. First of all, if you put them together as equals, the reader will wonder why you’re mentioning both statements at the same time:

Esther must do her homework now, but Mom is on the warpath.

This joining may mean that Mom is running around the house screaming at the top of her lungs. Although Esther has often managed to concentrate on her history homework while blasting heavy metal music at mirror-shattering levels, she finds that concentrating is impossible during Mom’s tantrums. Esther won’t get anything done until Mom settles down with a cup of tea. That’s one possible meaning of this joined sentence. But why leave your reader guessing? Try another joining:

Esther must do her homework now because Mom is on the warpath.

This sentence is much clearer: Esther’s mother got one of those little pink notes from the teacher (Number of missing homeworks: 323). Esther knows that if she wants to survive through high school graduation, she’d better get to work now. One more joining to check:

Mom is on the warpath because Esther must do her homework now.

Okay, in this version Esther’s mother has asked her daughter to clean the garage. She’s been asking Esther every day for the last two years. Now the health inspector is due and Mom’s really worried. But Esther told her that she couldn’t clean up now because she had to do her homework. World War III erupted immediately.

Do you see the power of these joining words? These conjunctions strongly influence the meanings of the sentences.