Between the three of us, its not rocket science

To address some very common grammar mistakes, here is a simple (and hopefully useful) list.  Did you catch the two errors in the title?
Its vs. It’s
This may seem self-explanatory, but it comes up all the time.  “It’s” is a shorter way of saying “it is.”  “Its” is possessive.  It’s a nice day.  The lion bared its teeth.
Between vs. Among
In addition to hearing this in casual conversation, I have seen this mistake published in books and heard it spoken in movies.  “Between” is defined as “at, into, or across the space separating (two objects or regions)” (underlining is my own).  If you are describing more than two, you need to use “among.”  Between the two of us; among the three/four/five/infinite number of us.
An example of how to use an apostrophe incorrectly.
An example of how to use an apostrophe incorrectly.
Using an apostrophe indicates possession. For example, that bat is David’s. This is a shorter way of saying that the bat belongs to David. We also use apostrophes to shorten “— is,” as shown in Its vs. It’s.  You can say David’s going to the park in place of David is going to the park.  The apostrophe is often added erroneously to words that are plural.  Ever seen a sign along the lines of, “We sell tire’s” ?  My family has received letters addressed to “The Tucker’s.”  Don’t do this.  It happens all the time,  but I repeat: don’t do it.  
More on apostrophes: There has been a lot of confusion about how to use apostrophes with words that already end with s.  If it’s singular possessive, like in the above example, where David is the singular person who possesses the bat, you add “ ’s.”  For example, Charles’s bat.  If it’s plural possessive, such as referring to an entire family by their last name, or any group by one title, you add an apostrophe to the existing s.  For example, the Smiths’ house.  A group of multiple dogs owns the bones?  Yes, those are the dogs’ bones.
Former vs. Latter
This is becoming less commonplace but it’s still useful to know.  Former and latter are ways to reference things you have already said so that you don’t have to say them again.  You usually use former and latter for a set of two things.  For example: Either you are doing this because you are upset, or because you’re trying to pull a prank on me.  If the former, I’m sorry I upset you, but if the latter, then bring it on.  Former always refers to the thing you said first, and latter to the next.  
Passive vs. Active Voice
This is useful for writing emails, proposals, etc.  It’s generally good practice to avoid using passive voice if possible.  Basically using active voice means that you emphasize the subject, while with passive voice the focus is on the action.  For example, I broke the glass, or, the glass was broken by me.  The former is an example of active voice because the subject is explicitly the one doing the action, and the latter shows passive voice because the emphasis is on the action being done to the object.  With passive voice you can also completely eliminate the subject, such as by saying, “Your car was stolen!” instead of “Darth Vader stole your car!”
Affect vs. Effect
“Affect” is a verb, “effect” is a noun.  The song has affected her, the song had a great effect on her.  Let’s look at the effects of climate change, let’s look at how climate change is affecting the planet.  
Me vs. I
The way to remember which one to use is to think which one would make sense by itself.  Him and me, or he and I?  This will depend on the sentence.  Give the pizza to me  give the pizza to him and me (or me and him, or us).  I want the pizza  he and I want the pizza.
i.e. vs. e.g.
This one may be less commonly used (I personally rarely use either of these), but it’s worth knowing.  i.e. means “that is” or “in other words” (similar to a.k.a), while e.g. means “example given” (i.e. “for example”).