Welcome to the forum!!!
I can only say that I wish my non-existent Spanish was as refined as your "rusty" English.
Here is an online soft version of the manual. Save it to your desktop and you can search it for items. I don't have the K500, but here are the explanations with references to the manual. The manual explains what the controls do. Unfortunately, it does not explain why you want to use them, or in what circumstances that they would be hand to be used in. Just remember that there is no right or wrong. Using the camera "wrong" can result in some really nice images.
So, to your questions:
1. How's the K-500?
The camera has the same sensor as the K5 K5II K5IIs, K30, K50 and K-01. It is a VERY good sensor. Both Sony and Nikon have used the sensor - but not on their entry level bodies - on their top of the line bodies. The K500 is an entry level body, so there are some features missing (creature comforts), but all the necessities are there for you. This is equivalent to a mid level camera for Nikon and Canon. How do you find it in quality, what should I know about it, tips, do's and dont's, any comment helps to get to know my new camera.
The sensor quality of the camera is VERY good. There is NOTHING you can't do with it. You are only limited by your own imagination. The image quality is superb. Here is another thread with some images (same sensor as yours). Also, just one item - they were taken at night - pitch black. The K500 can do the same (other than use the O-GPS1 attachment for startracking).There really are no do's or don'ts - go out and take pictures - any pictures, anywhere. You can't hurt the camera. The film is free. Have an idea - try it out. Sitting in the livingroom at night - with a few lights on, pull out the camera, crank up the ISO to 6400 and take a picture - to see what it looks like. The camera body is set up for you the photographer to have as much control as possible - so things are there for you to adjust and shoot. So, do this. On a weekend - go out and shoot anywhere - in the house, outside, in the park - where ever. Come back load the images up in your PC. Take a look at them and see what YOU do and don't like. Figure out what you want to change - and out and try again. Try a bit of everything. Landscapes, cityscapes - night lights, sit the camera on a wall or what ever to hold it steady, and press the shutter. Put it in manual mode and take a 15 second, 20 and 30 second exposure. Experiment.
2. What do I do with the mode "wheel"? I've taken an introductory class to photography so I know the basics regarding aperture, shutter speed, ISO and exposure. What I don't know is what this different modes offer and how I can get the most of them.
(page 40) - This wheel operates a bit differently depending on the model of camera you have. The K500 is a bit unique in that it allows you to select the mode from the menu being shown on the rear monitor. You are going to have to play with this a bit to understand, however think of a drop down menu - and this is the way you select the option within the drop down menu.
3. What's the green button for?
A bit of history. Pentax is famous for being able to let any old Pentax lens be mounted - even their old M42 screw mount lenses (with an adapter). These older lenses do not have the newer modern interfaces, thus you need to supply some of that intelligence. Lets say you have an old Pentax lens that you mount. This lens has an aperture ring (this is how you set the aperture on the old lenses). So you twist the aperture ring to say f8. Now you want the camera to meter the shot - i.e., apply the light meter and determine the appropriate shutter speed or exposure - so you push the green button. Its that simple. And the RAW/Fx button on the side?
(page 165) RAW/fx - there are two ways (or formats) that the camera saves images - RAW and JPG format. RAW is the data directly from the sensor, and provides more information for post processing. Where as in JPG, the camera takes the data from the sensor, and passes it through the incamera JPG engine, and they applies post processing on the fly. If you shoot in RAW then you have more latitude in terms of how much post processing can be applied to the final image. So, with that background what the RAW/Fx button does is to allow you to push it once, and it will save the image that your are taking in BOTH RAW and JPG formats (2 separate image files). There are reasons for this - like you wanting to see what the camera thinks the post processing should be, and you have the RAW file to do what every you wish with the post processing.
4. There's a switch with 3 options at the side of the camera (AF.S, C, MF) Which I'm guessing are for focus, what to these do?
(page 101) -This is how you tell the camera body that you want it to either use AutoFocus (AF.S or AF.C) or Manual Focus (MF). Lets say that you have the kit lens that is auto focus, so most of the time you have it in either AF.S or AF.C (more on this later). You want to take a picture but the camera finds it too dark out to be able to autofocus - it hunts back and forth, so you flip the switch into MF, and manual focus the shot. Now to AF.S or AF.C - There is a lot to this, but let me say this - AF.S - think static. A car is coming towards you. You have it in AF.S and you take a series of images. The camera will lock focus once and take a picture and if you have it in burst mode it will keep taking pictures. If however you have it in AF.C then as the car comes towards you, the camera body will focus, take a picture, then refocus, take the next picture, and keep going like this.
In answer to your first question - do and don'ts - there are none. Most folks would not take a 5 second exposure of something moving - as it turns out blurry - but sometimes, it turns out pretty well...
The forum is filled with wonder folks - who will usually take the time and answer questions. So just ask - there is no such thing as a stupid question - just a question not asked.
_____________________
The Pentax manual as I said above describes what the functions do, and does not describe why you may want to use them or not or in what situations. There is a book (an ebook) that is pretty cheap, that does a pretty good job filling in the rest - the why's and where's ---
_____________________
You did not ask - however, I am going to throw my two cents in anyway. Photography can be very capital intensive. You have the camera and a lens, the battery that came with the body, and you have an SD card. You are all set. You really do not need anything else. Go out and shoot with what you have for several months. See what you like and what you don't like. Experiment. You can spend a lot of money getting this and that, and perhaps not using half of what you pick up because you wind up not using it or liking to do something else. After a while, you will start to see what you are interested in and from there what additional items you MAY want to get (or not). Just remember - your first 10,000 images are the worst. It can be frustrating - especially after shooting with a point and shoot. Point and shoot cameras are designed to give you good images no matter what. dSLRs are designed to give you the photographer the control to do just about anything - and that includes taking some possibly really bad images. Just don't get discouraged. If it feels too daunting - just turn the mode button (on top of the camera) the the green
AUTO mode- and the camera will work like a point and shoot.