shall 是過時的英語。太文了。 你可以說 Tomorrow, I shall buy some vegetableS from the market. OR Tomorrow, I WILL BUY some vegetableS from the market. 口語從未說shall

You can say "I will", but it is a different expression. You say "I will": 1. When you make up your mind on the spot. E.g. I'll get some vegetables then. 2. When you are waiting to make up you mind. E.g. If it rains tomorrow I won't go. The example phrases you have given are kind of dead, as they don't really mean anything when not in context.

shall= you think it would be good to buy vegs tomorrow will=you are going to buy them tomorow for sure