structure of sentence

Question

Why is it 'on en achètera une fois qu'on sera arrivés" and Not 'on en achètera une fois qu'on arrivera"

Accepted Answer

Hi Shashank,
In French, we pay a lot more attention to what we call 'concordance des temps' than in English and this sometimes can create problems for the learner - 
In this example if you use the same tense for both actions, to buy and to arrive it means they are simultaneous which is not the case - you have to get there and then you buy the sun cream. It might be the day after or the same day but you can't do both at the same time.
So the future action of getting there is before the second future action of buying the cream which means you have to use the futur antérieur for the first action in time and the futur for the second action on the timeline.
A bit tricky to explain so take a look at the following lesson for more information -
Le Futur Antérieur (Indicatif)
 
Bonne Continuation !

Answer

Actually, in proper (high-register) English, you also use future perfect tense: We will buy one once we will have arrived there.

Answer

Perhaps English is more flexible in the sequence of tenses than French, but it has its own rules. You definitely cannot use the future perfect tense here, Chris.