Different words and phrases
I have two questions. First, how does one practice with different phrases that express the same thing? I just type the one that makes sense intuitively and comes to mind. The problem I think is that native speakers might use different phrases and I'd be out of the loop. Do you just force yourself to type one of the alternate phrases? My second question is about when you mouse over a certain word and multiple words in the target language come up. I'm sure it depends on context sometimes but it seems that the lesson always wants the first word that comes up. Is this context dependent or is it arbitrary? Thanks.
That is a very good question and a kind of difficult answer. I know exactly what you mean, and sometimes I struggle with the same thing. First of all, these words can be from different dialects of Spanish. Duo sometimes incorporates different words that are all correct but are more and less common in different regions. For example, the word for block ( like the school is one block away from my house) can be either "cuadra" if you are in Mexico or "manzana" (yes the same word as apple) if you are in Spain. Being aware of the origin of these words can help you avoid being out of the loop when talking to a native speaker. Sometimes might have slightly different meanings but mostly mean the same thing. This is tricky because you have to know which one fits. This you just have to practice. Try wordreference.com or spanishdict.com to find the exact meanings. I trust and use these websites frequently and they really help solidify the difference between two variations of word. I hope this answers your first question.
To dress your second question, it is context dependent but not rigid. There are times when a sentence might have a more straightforward answer with a clear separation between correct and incorrect but most of the time there are several words that are acceptable. Also the word you choose should match the intended meaning so if you want to say "The spider is huge!" you would translate "huge" directly to "enorme" and say "La arana es enorme!" but if you wanted to make a gereral remark about the large size of the spider, you could choose from a number of words such as enorme, gigantesca, grande, or masiva.
I hope this helps you! As I am still a student myself, I struggle with this alot and have to consciously think about every alternate meaning before I answer, but these tips always help me. If my answer is unclear or confusing please let me know and I can try again.