The weather forecast says it will get warmer when we arrive in Beijing.

Represents the future tense, be

Raining stands for a continuous process, and when-guided clause stands for a specific time point.

I

have

arrive

be

leave

you

exist

that

Instantly. I will have to leave you.

A

Grammatically, will is followed by the prototype of the verb. If it's Will,

Rain, there can't be a when clause, because it represents the future tense in a general sense, not a specific time point, but it would be nice if it were tomorrow.

C

Why not use c for continuous or future tense? The future tense is generally used only in spoken English, and it is used alone, that is, there is no clause behind it, which does not meet the conditions for use. Like his.

Go ahead. He's leaving.

D

Would stands for hypothetical type, which is only a statement of future situation, and does not involve the meaning of hypothesis or guess.

weather

report

What did you change it to?

Say is useful for the need of virtual form, and would can also be used as a euphemism to express mood, but it is not suitable for this sentence, it can also be used as the past tense, and it is not suitable for this sentence.

Reference Baidu.