Structure Skill 5 : Coordinate Connectors

Many sentences in English have more than one clause. (A clause is a group of words containing a subject and a verb.) When you have two clauses in an English sentence, you must connect the two clauses correctly. One way to connect two clauses is to use and, but, or, or so between the clauses.

The sun was shining and the sky was blue.

The sky was blue, but it was very cold.

It may rain tonight, or it may be clear.

It was raining outside, so I took my umbrella.

In each of these examples, there are two clauses that are correctly joined with a coordinate connector—and, but, or, or so—and a comma (,).

Example:

I forgot my coat, ____ I got very cold.

(A) then
(B) so
(C) later
(D) as a result

In this example, you should notice quickly that there are two clauses, I forgot my coat and I got very cold. This sentence needs a connector to join the two clauses. Then, later, and as a result are not connectors, so answers (A), (C), and (D) are not correct. The best answer is answer (B) because so can connect two clauses in this manner.

The following chart lists the coordinate connectors and the sentence pattern used with them.

Source :  http://www.toeflskill.com/2011/02/skill-5-coordinate-connector.html