"Mom and dad are in the park."
Translation:Мама и папа в парке.
The most practical way is, unfortunately, to memorize which preposition is associated with a noun when you are "at" that place. The overall rules do not work for each noun, especially if the technology moved on (a subway station is obviously an enclosed space, but you will still be waiting "на станции").
No, place does not always come first in Russian. First comes what's already known, and the last comes what's important. Here, the question is "Where are Mom and Dad?", so the answer to the "where" comes last. If you ask "Who is in the park?", the answer to this "who" (Mom and Dad) will come last.
If you have to connect two items into a list, you use и.
А is used for juxtaposition of two things or correcting a "false" fact by providing the correct description:
- Мама в парке, а я в школе. = Mom is at the park and I am at school.
- Я не в парке, а дома = I am not at the park but rather at home.
- Я в парке, а не дома! = I am at the park, not at home!
- Папа ест апельсины, а я нет. = Dad eats oranges and I do not.
А is used in "And you?" questions:
- Я буду на работе. А ты? ~ I'll be at work. And you?
- Так ты дома? А Маша? ~ So you are at home? And what about Masha?
It is also used to push narrative forward (just saying: you might encounter it as you read more authentic texts):
- Ира осталась дома. На улице дождь, а в дождь у неё болит голова. ~ Ira stayed home. It is raining outside, and she gets headaches during rain (lit. "head hurts during rain").
Russian Present Tense usually omits the verb “to be” ‧ www.duolingo.com/skill/ru/Alphabet-1/tips-and-notes
In Russian the copula быть (byt’) is normally omitted in the present tense, but not in the past and future tenses ‧ zero copula is common in some Vernacular English ‧ en.wikipedia.org/wiki/Zero_copula ‧