Spanish language Q&A Forum

I was doing a writing exercise, and I put "estar por" instead of "estar a punto de" and I was corrected and "estar por" wasn't shown as an option. I'm wondering why since I have heard and used this expression frequently. 

The phrase that has to be translated was, "He was about to go out the door when..."

Ella se mudó allí hace dos meses.   Is this correct?  The lesson says we can't use the preterite with hace + time.

Hola Señora,

Dónde tenemos que usar SOBRE y EN por ON en español.

la chica que le gusta - is ambivalent. It can mean both "the girl he likes" and "the girl who likes him"?

What is the difference between:
ser + [noun]

and 

ser un + [noun]

What is the use case for each?

Why is the last sentence written as "y no abusemos de ellas"? I thought that it should be "no las abusemos". Is there a rule for when you put "de ellas" at the end of the sentence? I thought that you weren't allowed to have pronouns on the end of sentences in Spanish. 

Oh I'm trying so hard, but I'll be 103 years old before I understand whether to use the imperfect or the simple past!

Hola Inma y equipo, 

As part of the 'All related grammar and vocab' list for this exercise, would I be right in saying that Inma's excellent lesson, that I've included here, should also be listed?

Using tener + participio to express the completion of an action (perífrasis verbal). 

Gracias

Clara