Verbs of position
I am writing a little introduction to a topic that baffles a lot of students of Spanish: verbs of position in Spanish.
First let's look at what a past participle does. Take a verb like "to break", which is a verb of (almost instantaneous) action. The action takes place only while you are exerting energy on an object and this breaks apart. Once the object is reduced to pieces, the act of breaking the object is over. Now the object is broken (not breaking). The past participle "broken" indicates the result of a finished action.
Now let's look at the verb to sit in English first (from the Cambridge dictionary):
sit to (cause someone to) be in a position in which the lower part of the body is resting on a seat or other type of support, with the upper part of the body vertical. Eg. to sit at a table/desk
The fundamental difference between this verb and the Spanish "sentar(se)" is that in English the verb can be regarded both as a change of position and an ongoing action (to continue to be in that position), whereas in Spanish "sentar(se)" is only the act of moving your body so that the lower part of it eventually rests on a surface. As soon as you touch the surface, the action of "sentarse" is over, like in the example breaking the object. Once the actions are over, the object is broken (you are not longer breaking anything) and the person is "sentada" (you are not longer "sentándote"). In English you are sitting, which means that it is an ongoing action, which consists simply on remaining in that position, whereas in Spanish your position is just the result of a completed action (sentarse) that lasts less than a second on average. Unless you move, the result of your finished action is to remain in that position (the broken object also remains in that state). In other words, after you "te sientas", you begin to be "sitting" there. Two different verbs. A sentence like "Estoy sentándome" (literally "I am sitting") does not mean that you just stay seated, but it describes a one-second action as your body moves a few inches towards the surface you'll rest on - it turns a very short action into a slow motion movie of it.
The same goes for many other position verbs, like echarse, acostarse, tenderse and tumbarse (to lie). In English they refer both to move into a horizontal position on a surface and to remain in that position, whereas in Spanish they only refer to the change of position. Again, they are all actions that rarely take more than a couple of seconds (unless you have mobility problems). Once your reach that position, you are not longer "acostándote" (lying), but "acostado", which is the result of the finished action.
Other verbs like "agacharse" are the same, but interestingly, while "to squat", which means "agacharse" behaves like "to sit" in the sense that you are squatting when you remain static, "to bend", which also means "agacharse", behaves like "agacharse" in the sense that it is only an action (a movement), and once you reach your final position you are no longer bending - now you are bent (over), you are "agachado". In Spanish once you reach your final position, you are bent, squatted, bowed, ducked,...
Add to this list other similar ones like arrodillarse (to kneel down), encorvarse, inclinarse, agazaparse, acurrucarse, encogerse, doblarse, torcerse, girarse, apoyarse,...
Only verbs like "yacer" (which is too formal and often used with corpses) are used for states, so "yacer" (to lie) is to remain in that position, like in English, but in Spanish, you normally cannot use the continuous tenses with verbs of state.
It is also worth noticing that "yacer", the only one that is different from all the others, is the only one without "se" at the end. This is no coincidence. All the above verbs without "se" would be transitive verbs, so you'd perform the action on other things, people or parts of your body: you'd make people sitting, you'd make people lie down, you'd bend them, bring them down to their knees,... The "se" indicates that you are not doing it to other people, but simply doing it (intransitive). Unlike normal reflexive constructions, you cannot say "a mí mismo", because you are not doing it to yourself, but just doing it.
19 Answers
If anyone has some topic like this in mind, please let me know and I'll give it a go. I welcome challenges (but I don't promise anything).
For those with a vivid imagination (or an artistic talent), imagine all these explanations: the form "breaking" can be visualized showing an object just disintegrating, and a broken object as a set of broken pieces. Similarly, "sentarse" could be pictured as a person slowly approaching the chair (or any other resting surface), and a person on the chair viewed as "sentada". These explanations are optimal when you can "see" them (that's why comics are so useful).
I am glad people appreciated these insights. I learnt about them during my endless research readings, and I found the phenomenon very revealing, so I wanted to share all those insights with you.
Great explanation as usual from Lazarus.
I would just add something. "I am sitting" in English = "I am sat" - in other words the "sitting" is being used as an adjective and not as a present continuous verb form.
I hope that doesn't confuse the explanation.
Lazarus. You have a talent and should be earning lots of money from it. I have read, listened to and watched many different Spanish learning materials, but nothing comes close to your ability to get the points across. Perhaps its just the way I learn, but I really understand things when you explain them.
Thank you so much!
Thank you so much for taking the time to do this.
Definitely helpful in seeing how differently the English mind and the Spanish mind perceive progressive actions.
It brings to mind another question regarding SE (aughh), which I have somewhat gotten used to, but still hate because it seems confusing to me. Maybe I should open another thread about it though, because it is very off-topic here.
Lazarus, gracias! I have read so many of your posts and am truly amazed at your explanations. I even thought you were a professor of the language and asked one of the administrators who said no that you just had a passion for teaching. Again, this is a great lesson.
Fantastic Lazarus, glad I popped in for a short time and didn't miss it. :D
I have made a SpanishDict flashcard set with the verbs that Lazarus has discussed in this post.
You can find it under the Flashcard Tab and it's called "Verbs of Position".
Again, thank you Lazarus.
Going with your last point, It seem like reflexive pronoun are used because your preform this actions yourself and not someone else.
Excellent, as always. That clears things up considerably.
Lazarus - you have a very special talent. A hearty thank you.
but in Spanish, you normally cannot use the continuous tenses with verbs of state.
Thank you, Lazarus! That was the light bulb going on for me. I think I finally understand.
Once again, I wish I could vote twice for you! 
Wow!...and now I have copied out all those words into a list to learn...including "yacer" :- ) Gracias!
Thank you, again, for the explanation. I find myself learning more and more, and having a little more ability to actually understand more, everyday. Posts like these are so helpful!! 
