Do not join independent clauses with a comma. If you want to join complete sentences, join them with either a semicolon or a conjuction. "And," "but," and "or," are the most common conjunctions.
A comet struck the earth about 65 million years ago near what is now the Yucatan Peninsula, it caused the last of the dinosaurs to go extinct.
Both clauses in this example are grammatically complete. Use a semicolon instead of a comma if you want to write them as a single sentence. You could also rewrite it as two sentences:
A comet struck the earth about 65 million years ago near what is now the Yucatan Peninsula. It caused the last of the dinosaurs to go extinct.
or as a single sentence in which the clauses are joined by a conjunction:
A comet struck the earth about 65 million years ago near what is now the Yucatan Peninsula, and it caused the last of the dinosaurs to go extinct.
When a semicolon is used to join two independent clauses it suggests a closer relationship than when they are treated as separate sentences. Suggesting this close relationship is especially useful when the first clause gives the cause for the effect described in the second (Strunk and White 2000).
The number of the subject determines the number of the verb. Don't be fooled by words that come between the subject and the verb. You wouldn't write:
The flower are well adapted for pollination by hummingbirds.
So don't write:
The flower with its elaborate mechanisms for gradually dispensing pollen, copious nectar, and brilliant, red corolla are well adapted for pollination by hummingbirds.
To follow this rule you have to know what the subject is. The plural "have" appears in the following sentence instead of the singular "has" because "scientists" is the subject of the phrase "scientists who have studied this problem."
She is one of the most talented scientists who have studied this problem.
One more thing to remember: "data" is plural. Write "these data are" not "this data is " (Strunk and White 2000).
Make sure that a phrase at the beginning of a sentence referw to the subject of the sentence.
Being unable to fly, an ornithologist has little difficulty collecting altricial chicks from an unguarded nest.
The phrase "being unable to fly" refers to the chicks, not to an ornithologist. This sentence should be rewritten as:
Being unable to fly, altricial chicks are easily collected from an unguarded nest.