Hi Helen i prefer to use "will go to swim" according to the sentence, You can have conditional type 1 to answer this. ( if + simple present tense will get "simple future as a result ) "will is different with going to" will = scheduled going to = unplanned So, when you predict something happen in the future, you must be have a plan to do it. Hopefully it's helpful for you. Add me as your friend when you really being helped with my explanation. cheers

You could phrase it in the ways you have suggested, but it would be more natural to add 'go' and use swim in the gerund form: If it is sunny tomorrow, we will go swimming If it is sunny tomorrow, we are going to go swimming

Thanks, Lydia!!!

Hi Helen, actually Paulus' answer was the correct one. You can only use "we will" not "we are going to" in this case. This web page might be helpful: http://www.perfect-english-grammar.com/conditionals.html