in which tence we use shall do 

give reason

i think its future tense as will and shall are the helping verb of future tense...

it is future tense .we use will and shall in it

 We use will & shall in future tense as they both are the helping verb of future tense.

we use shall in future tense as we are noy sure about the time when we are executing our work

 WE US SHALL IN FUTURE TENSE.

we use shall is future tense generaly we use will but in certain sectuation we have to use shall