Will | Shall
Will and shall are what grammarians call modal auxilliary verbs, but we will call them helping verbs, since that is what they do. Helping verbs lack conjugations of their own, but help other verbs by adding extra depth to them.
The traditional rules state that you should use shall only when refering to the first person (I, we), and will should be used when referring to the second and third persons (you, he/she/it, they). However, the use of shall is far more common in British English than American English, where it can be seen as pretentious, with a few exceptions for fixed expressions, such as we shall overcome. Nevertheless, knowing the subtle difference between the two verbs can help you to decide whether shall or will is more appropriate in your particular sentence.
Whilst will and shall both introduce the future tense, they indicate slightly different intentions as to what is expected to happen:
- Will implies intention; the speaker or writer intends strongly to carry out the action
- Shall implies necessity; the speaker or writer is definitely going to carry out the action
Generally speaking, it is better to follow the traditional rules - "I shall leave at 9am tomorrow, you will follow me later". Although both have the air of a command, will leaves some room for the other person to make the decision for themselves. It is possible to get things very wrong, however, as this joke illustrates:
A foreign tourist was swimming in an English lake. Taken by cramps, he began to sink. He called out for help:
“Attention! Attention! I will drown and no one shall save me!”
Many people were within earshot, but, being well-brought up Englishmen and women, they honoured his wishes and permitted him to drown.
You may, in the end, find it simpler and less prone to misinterpretation to use another auxilary such as must or have to, which will convey the necessity implied by shall with far less ambiguity, and are far more commonly used.