To go for a picnic is when You're planning it yourselves, maybe with your frienda. To go to a picnic when It's already happening out there, and you wanna go. To go on a picnic is like considering a long picnic/considering the picnic like a vacation or long distance travel, like You're going on a vacation/tour that has already been planned.

To go for a picnic is best, ‘to’ can be used in certain contexts, normally if someone else has organised the event and you are attending, rather than preparing your own

hi I am younes 23 years old I speak english german french arabic amazigh and i am intermediate in spanish i like learning languages discover new cultures read books practice sport i would to practice with you if you could practice any of these languages i f you like thanks

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