Spanish language Q&A Forum

I have taken this test several times. Each time I check   Comprad tanto carne como pescado,  it marks me as choosing tanta.  

Why is it not correct to use El Subjuntivo de Presente here, when the sentence starts with creo que? All the possible answers used Futuro instead.

       I hope this might serve a beneficial purpose. I just had a conversation with someone in México (also a degreed Spanish teacher). During the conversation I thought I would tryout the new phrase I learned here; I used the phrase "Estar deseando + infinitive in our conversation: "Hija, estoy deseando pasar tiempo contigo en La Navidad."  She advised that, while she understood what I was saying, it is not commonly used there and it sounded a bit odd, as if I was translating exactly from English. She wondered if was mostly used in Spain. She advised that Spanish speakers in México are more likely to use - admittedly, colloquially-  "Ya + verb Querer:"Ya quiero que pase tiempo contigo en La Navidad." This translates to: "I am looking forward spending time with you at Christmas."   As for the phrase "Tener ganas," she agreed that it could mean " looking forward to," but in México it is more associated with " I feel like ( doing/ having something)."

From the examples given it's hard to tell why a native speaker would choose one of these adverbs in preference to another one in any given situation. Is there any guidance on this? Why would I choose 'igual' over 'lo mismo', for example, if they both mean the same thing?