1, 2 and 4 are correct. In 3, traditionally it would be more proper to use “were” instead of “was”; this is called the subjunctive mood, and it is used to talk about unreal or hypothetical situations (such as discussing what you would have done if the weather were sunny, when in fact it was not sunny). That said, many American English speakers no longer use the subjunctive. In 5, “was” would be better as were (for the same reason as #3) or as “had been.”

Hi Anna, For #1 especially, no “weather”. It is natural to just say “it is sunny”, and speakers know you mean the weather. For #5, it seems like the meaning is “we are talking in the evening about the day just ending”, so I would use the past perfect for the condition clause, just like in #4. So, “if it had been sunny today, we would have gone to the sea.” We can drop “weather” and use the placeholder subject “it” to sound more natural. Edit: John is right about the subjunctive, but it might sound formal here.

In speech, use it’s we’d we’ll. Beach/coast/shore instead of “sea” (in the US). If the weather’s good/bad If it’s sunny/rainy If it’s sunny today, we’ll go to the beach.