Comma before and after conjunctions

Comma after so
When so is used to mean therefore, it is a transitional adverb. So, it should be followed by a comma.

• I was not feeling well. So, I decided to consult a doctor.

Semicolon before And
A coordinating conjunction that merges two clauses into one is usually preceded by a comma. However, the comma can be left out when the clauses connected by the conjunction are very short.

• The boys sang, and the girls danced.

The comma is not exactly necessary in the example given above because the clauses are short. However, it is possible.

If one or both of the clauses connected by a conjunction contains a comma, we sometimes use a semicolon instead of a comma. Note that although a semicolon is possible here, it may look dated.

• Alan, Sophie and Mira absolutely love the steak pie; but Maria, a staunch vegetarian, detests it.

Here the two clauses are connected by the coordinating conjunction but. However, we separate them using a semicolon because the second clause contains two commas.

Note that a semicolon is not exactly necessary here. A comma will work just fine. In fact, a comma tends to be more common than a semicolon.

• Alan, Sophie and Mira absolutely love the steak pie, but Maria, a staunch vegetarian, detests it.

That-clauses
Commas are not used before that-clauses.

She said that she would come. (NOT She said, that she would come.)

Notes
A that-clause is essentially a noun clause that serves as the subject or object of the verb in the main clause.

Comma before subordinate clauses
When subordinate clauses come at the beginning of sentences we usually separate them with a comma. The comma can be left out when the subordinate clause goes after the main clause.

Because he had been reading my letters without my knowledge, I was quite angry with him.

I was quite angry with him because he had been reading my letters without my knowledge.