Spanish language Q&A Forum

I think I've managed to wrap my head around how the passive works in a basic sense, but I'm wondering if anyone can offer, or refer me to, any guidance on WHEN to use different passive/impersonal forms, or how the nuances change? I know this is a rather broad question, so I'll try to narrow it down to a couple examples:

When is it prefered to use the true passive versus the se refleja form? for example, I was reading an article that said "las piedras habían sido extraídas de rocas que se formaron hace miles de millones de años." Here we have two different forms used in the same sentence! Could the writer have instead said "las piedras se habían extraído de rocas que fueron formado"--or some other combination--and if so are there different nuances?! Is one simply more formal? Or is there another specific reason the se pasiva wasn't use for one but it was used for the other? 

Also, I know this is a lot at once, but I'm struggling to grasp how the use of the passive with "se" differs from the use of the "ellos" impersonal construction. For example, if a house is under construction down the street, would you say "se construye una casa" or "construyen una casa" and if both are equally valid, how are the nuances different? And are there cases where one is possible but the other isn't? For instance, I've often noticed that when the object of an action is a person rather than a thing the action is often not expressed with se--the ellos form seems to be the choice in some cases like "le robaron" (but not "se robó"?). And yet... we do have "se buscan secretarias"? I can't quite see what is going on here...

Mil gracias in advance for any help on any of these questions... 

Why isn't "las" used to modify both bicicletas and cintas de correr? I thought bicycles and treadmills were a general class of equipment, and not a specific bike or a specific treadmill. Thank you. 

So ¿cómo está? is an incorrect answer for a child?

Don't remember ever hearing that one, only that it is okay to use the informal version with them.

Also you site is jumping past questions when I give the answer to a previous question, I know I should notice but your site shouldn't do that either.

It's happened a couple of time and I think I've done well on a test only to find out I didn't answer a couple of the questions.

Also there are drop downs that don't show some of the answer and lo and behold one of them is apparently the correct answer, which only confuses me because I end up picking the best sounding incorrect answer.

I was marked wrong for using “habían” when the sentence was something like “habían muchos mensajes …”. If “había” is used for one thing or many things when would you use “habían”? It sounds more natural to say “habían muchos mensajes …”

I have done two tests now where I have chosen "estaba imprimido", but it tells me I've chosen "estaba imprimiendo", so it is counted as an error.  I don't know why that is happening.  

"Su familia la quiere mucho y, para ser sinceros, ha hecho muchos sacrificios por sus hijos." quote from cloze exercise

I do not understand to whom 'para ser sinceros' refers to.  Does it refer to 'su familia' or Eva, the daughter who made the sacrifices? 

Why is 'sinceros' plural since it refers to a singular noun, either su familia or Eva?

This sentence doesn't make much sense to me.

I understand the need for para in para ser sinceros to mean in order to be sincere, but I am confused as to why sinceros is plural and to whom it refers.

Thank you.

 Using para (and not por) for purpose, goal or objective