Spanish language Q&A Forum

I am not clear on when to use which term.  In the passage, there is "el esfuerzo medioambiental" and also "modelo ambiental".  Are they interchangeable?  Thanks.

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.

I wish there were a lesson explaining how the use of articles in Spanish differs from English.  For example, "he has a good heart" is "tiene buen corazon" not "un buen corazon".  Another example, "I will be in the first row" is "estare en primera fila" and not "en la primera fila".

Me cancelaron el viaje, cosa que me fastidió mucho.

They cancelled my trip, which annoyed me a lot.

this sentence uses cosa que in the middle of the sentence

the notes state otherwise using la cosa que and it says only for beginning of the sentence not middle

am i right to conclude, cosa que = lo que in the middle of the sentence, whereas la cosa que = lo que in the beginning of the sentence.

can i also further conclude below

Le encantan los documentales de historia, cosa que/que/lo que/lo cual (yo) encuentro aburrido = I find [the fact] that she loves history documentaries boring.

cosa que/que/lo que/lo cual are all interchangeable and replacement with no change in the meaning in the middle of the sentence?

la cosa que/lo que for the beginning of the sentence.

to summarise some of the info provided in the comments.

am i right to say the below:

Le encantan los documentales de historia, que/lo que/lo cual (yo) encuentro aburrido = I find [the fact] that she loves history documentaries boring.

Le encantan los documentales de historia, que/los que/los cuales (yo) encuentro aburridoS (agreement of the adjective is necessary here) = I find history documentaries boring.

meaning to say, que = lo que = lo cual are interchangeable and replaceable with no change in meaning. same for que = los que = los cuales ? 

Hoy invito yo, así que escoged ________ os guste.

This sentence has subjunctive so am i right to say it means = choose whatever you like?

if it means = whatever you like, then why is cual or cuales wrong for this sentence?

thanks

under this section, the quiz showed a question as below.

Estoy intentando que ellos nos ________ una oportunidad.

i am not sure why does intentar trigger subjunctive.

is this because it contains uncertainty? so if im using other similar verbs such as probar or whatever scenario of trying to do something, i will need to use subjunctive?

I'm not sure I understand why the usage is "mensajes clave" vs. "mensajes claves"

Is clave in this context an adjective?  If so, shouldn't it agree in number with the noun?
mensajes clave vs. mensajes claves

As an example, wouldn't  "important messages" be plural? e.g. "mensajes importantes"? 
Why does "clave" stay singular when "mensajes" is plural?

I must have missed this grammar lesson on adjectives like Clave :-)

When talking about past actions we can also use it with the subjunctive, but this makes it sound more formal.