Spanish language Q&A Forum

Hi there, I have (belatedly) discovered Notebooks and am finding these a great way to test myself across different levels and hone in on my weak points. I've added a load of lessons, but it feels like it's focussing very much on the higher level (B2) ones. I want to interleave my practice with my weak points from A2 and B1 too. 

Question: how does it decide which lessons to test from a notebook? Is it random? Or based on your current %? Or something else? 

Also, I'd LOVE a feature where it just quizzes you from your weakest lessons *of those you have already been quizzed on*, across all levels. Basically that's what I'm trying to achieve. 

Can I use both por and con el ruido. Is there any difference between them?

Can I use mi pierna? In other lessor it was siad that it should ne la pierna. When can I use pronoun with body?

Here is another example of the nonuse of an article in Spanish that I do not understand.  "The city was an environmental model" is "la ciudad fue modelo ambiental" not "un modelo ambiental".  I do not understand why there is no indefinite article like there is in English.

How do learners remember when to use the "de" described above? Do native speakers learn it only from growing up with the language? I don't see any particular rule for knowing when to use the "de" and when not to--how "wrong" is it to omit the "de?"