Questions & Answers about Летом мы поедем либо на море, либо в горы.
The current lesson is about то ли ..., то ли .... Could we use that here instead of либо ..., либо ...?
They express completely different ideas. либо ..., либо ... is for a deliberate choice between two options ('either X or Y'). то ли ..., то ли ... expresses doubt, guessing, or a hazy memory. If you said Летом мы поедем то ли на море, то ли в горы, it would mean 'We are going somewhere, but I can't quite remember if we decided on the sea or the mountains.'
Why do we use the preposition на for the sea but в for the mountains?
This is a fixed pairing in Russian. You go на море (to the sea/seaside) but в горы (to the mountains). Because the verb поедем shows active motion to a destination, both nouns take the accusative case here.
Why use the perfective future поедем rather than the imperfective будем ездить?
You use the perfective поедем because you are talking about making a single, complete trip this coming summer. If you said мы будем ездить, it would imply you plan to make multiple, recurring trips to those places throughout the summer.
Just to check my memory, why is Летом in the instrumental case?
Russian frequently uses the instrumental case for seasons and times of day to turn them into adverbs answering the question 'when?'. That is why the noun лето (summer) becomes летом (in the summer), just like утро becomes утром.