The Cylon’s saw there prey over their, they’re spines glowing in anticipation.

Ok, You deserved that one. Have a good cry.

Look, It’s easy. Apostrophe’s do two things in the English language:

• Show Possession
• Show Contractions

The Problem is they can’t do both at the same time

Apostrophes Show Possession

If the noun doesn’t end in s, add ’s, regardless of plurality

Baltar’s demons

The Cylon’s Basestar

The men’s triangle team

If the word ends in s, usually add just the apostrophe. Some style guides do show ’s for words ending in s, as that’s the way they’re still pronounced. Sorry, this is still in flux

Gaius’ fantasy

pilots’ Vipers (Pilots’s Vipers? Nahh…)

Compound words and joint ownership put the ’s at the ery end of the bunch of nouns:

Commander-in-Chief’s quarters

Starbuck and Apollo’s relationship

Six and Gaius’ destiny

Personal pronouns do not have the apostrophe : Indefinite pronouns do.

His, Her (hers), Your (Yours), Their (Theirs), Its

Everybody’s home planet

Contractions

While we’re on the subject, here’s where we start having problems. The apostrophe can contract "Is" and "are" down so they sound the same as the possessives:

Everybody’s home planet

Everybody’s going home

This leads to all kinds of homophones in English:

It’s looking at its former body.

They’re standing over there next to their ships.

You’re not getting your weapon back.

And in the heat of battle, typing away at your desk, it’s easy to miss these small mistakes. But editors, who are looking for any way they can to cull their paperwork, see these small, common mistakes, they make the snap decision that you are a small, common writer.

Go ahead, have that cry.