I'd rather write: If it's sunny tomorrow, we'll go fishing. Conditional construction with present goes better with the clause in future (will) If I win the lottery, I'll buy a house

Yes, both sentences are technically correct, if a little stilted. The 'It being sunny' phrase is correct but rather formal, and you need a future form to refer to the weather tomorrow. A more natural way of saying this is, for example, 'As it's going to be sunny tomorrow, we're going fishing.' I'd use the 'going to' future in the first clause, because there is evidence (from the meteorologists' point of view) that this is is going to happen. In the second clause, you could say 'going to go fishing', but it is a little wordy, so it's more natural to just say 'going xxxing', using a present continuous for a planned future activity. The 'will' future tends not to be used for plans. We use it mainly for - conditionals: 'If the it's sunny tomorrow, we'll go fishing.' - decisions and offers made at that moment: A: 'I want to try out my new fishing rod' B: 'OK, then. We'll go fishing tomorrow.'

Both are grammatically wrong. It should be, "Being sunny tomorrow, we will go fishing" or "Being sunny tomorrow, we will be going fishing". The first one is future tense and the 2nd one is future continuous tense. Future tense implies that you are planning or something is going to take place in the future but has not happened as of yet. Future continuous tense is that you are thinking of the future and act as if it is taking place right now.

It will be sunny tomorrow, we are going to go fishing. (you know it will be sunny tomorrow) Since it will be sunny tomorrow, we will go fishing. (since: you know, therefore) [It being sunny tomorrow, we will go fishing.] - It being sunny tomorrow, sounds like Since it will be sunny tomorrow He 'will' be back/He'll be back She 'will' be going now/She'll be going now Like how we have I'm for I am and He's for He is 彼が戻ってきます (google translate) will: want to(for something to happen), for something to happen in the future (future tense) going to: want to go to (also future tense) If you think about it, will can be used for more meanings Going to is directed towards somewhere, a place

You can say "we are going to go fishing" or "we will go fishing." It means the same thing, both are correct. However, I would change the beginning of the sentence to "If it is going to be sunny tomorrow" instead.