# punctuation - Should "So", "Therefore", "Hence", and "Thus" be followed by commas?

2014-07

Often, I have come across sentences that begin with "So". Should such an usage of "So" be followed by a comma?

Are the following examples correct.

He is very good at computers. So, I think he can fix your computer.

When we multiply an even number with another even number, the result is an even number. So, the square of an even number is an even number.

What happens if we choose to use "Therefore", "Hence", or "Thus" instead of "So"? Do the rules still remain the same?

When we multiply an even number with another even number, the result is an even number. Therefore, the square of an even number is an even number.

When we multiply an even number with another even number, the result is an even number. Hence, the square of an even number is an even number.

When we multiply an even number with another even number, the result is an even number. Thus, the square of an even number is an even number.

Consider the following phrase:

If the fruit is an apple, it should be given to children.

Is the comma after *apple* necessary? Is it optional? Is it wrong?