YOU SAID:

I will do that next week

INTO JAPANESE

私は来週それをします

BACK INTO ENGLISH

I'll do it next week.

INTO JAPANESE

私は来週それをやるつもりです。

BACK INTO ENGLISH

I'm going to do it next week.

INTO JAPANESE

私は来週それをするつもりです。

BACK INTO ENGLISH

I'm going to do it next week.