SIMPLE AND CONTINUOUS TENSES MADE EASY

A lot of beginning writers have difficulty with the difference between simple and continuous tenses. Once you understand the purpose of them, though, there's really no difficulty.

Starting with the past tense, as the distinction is the most readily apparent, let's consider the operation of simple and continuous past tense.

PAST SIMPLE: Rover ate a bone.

PAST CONTINUOUS: Rover was eating a bone.

In the simple past tense, the action described is complete. The entire action of Rover eating the bone all happens within the confines of the sentence. At the beginning of the following sentence, he has finished eating it. Of course, you might go on and describe him eating it, but that's in the nature of a recap (Rover ate the bone. He went at it like a mad creature, tearing and slobbering.) In the most simple narrative, the next sentence moves on in time. (Rover ate the bone. Then he had a big drink out of the water bowl.)

In contrast, in the continuous past tense, the action described is ongoing. At the end of the sentence, Rover is still eating away at that bone.

Thus we may see that the continuous past tense is used to describe a state of affairs. For example:

When I walked into the kitchen (simple past; action completed) I froze in horror. (simple past; action completed)
Rover was eating the Sunday roast, all over the floor. (see the difference? This is telling you what is now going on, and it's still happening). Fluffy was in the corner, vomiting. (again, she's still golloping away).

Now let's apply this to the present tense. As with the past tense, there are two forms of it.

PRESENT SIMPLE: Fluffy kills a seagull. 
PRESENT CONTINUOUS: Fluffy is killing a seagull.

In the present simple, the action, again, is complete. At the end of the sentence that is an ex-seagull. It has ceased to be. Only a carcass remains.
In the present continuous, what we have is a point-in-time snapshot. Fluffy is killing a seagull. Right here, right now, this is what's happening. Help! Get the broom! He's killing it! You still might save it if you act quickly because the act of killing is NOT COMPLETE.

A good rule is always to use the simpler form when it does what you want, reserving the more complex form for when the simple one doesn't work.

EG: I walk into the room. Rover eats the Sunday roast.
See how that completely doesn't work? Here, we are saying that Rover eats the roast, commencing his depredations only after you walk into the room. The two sentences look disconnected and weird.

I walk into the room. Rover is eating the Sunday roast.
See the difference? OMG what's he doing, that I JUST CAUGHT HIM AT!

This same distinction applies to every tense. A rule of thumb is that whenever you have the participle (eating, opening, whatever, things ending in -ing) you have a CONTINUOUS action. It is the same in the future. 

EG: When you next see me, I will drink coffee. (I will have a coffee with you even though I have not been doing so). 
When you next see me, I will be drinking coffee (I will be slugging down the black stuff at the moment you walk into the cafe).