I'll go to California next week.

[LE AVOCADO]

Hi teachers
I have asked many related post in many English forum, But I am still confused:

Most of grammar books say that simple future-will is used for spontaneous decision as following:

Future simple tense can be used in situations where we have no previously made-up plan, and we are making a decision on the spot while speaking.

Is this theory true?

If so , when I already decided a plan , then I want to tell my plan to my friend, I can't use simple future -will:

Context:
If two days ago, I already decided to go to California next week. This means that I already decided to go California. Now I meet my friend, I want to tell her about my upcoming trip and want to invite her to join

I 'll go to California next week. Would you like to join me?

Is it wrong to use I 'll go to .... in that context.

I have asked many native speakers, but the opinions differ. Some says that it is still fine to use I 'll go...

But some say that it is unnatural to use simple future here. This makes me confused.

 

[jutfrank]

Most of grammar books say that simple future-will is used for spontaneous decision as following:

Is this theory true?

It isn't a 'theory' but it is true, yes. This is just one of several uses of 'will'.

Is it wrong to use I 'll go to .... in that context.

Yes, it's wrong. Since going to California is a scheduled plan that you have made arrangements for, you should use the present continuous tense:

I'm going to California next week. Would you like to come with me?

I have asked many native speakers, but the opinions differ. Some says that it is still fine to use I 'll go...

But some say that it is unnatural to use simple future here. This makes me confused.

You'll get better answers if you ask good English teachers.

 

[LE AVOCADO]

This picture is from English Grammar in Use, a reliable grammar book.
The book says that we can use simple future- will for something that is planned. Do you agree? I think that we shouldn't use simple future- will for something that is planned.

 

[5jj]

Most of grammar books ...

Omit 'of'.

 

[teechar]

I have asked many related posts in many English forums, but I am still confused

See above.

Most of grammar books say that ...

or

Most of the grammar books that I've read say that ...

See the following links to learn about "most/most of" (which also applies to "some/some of":

Some

Some - English Grammar Today - a reference to written and spoken English grammar and usage - Cambridge Dictionary

dictionary.cambridge.org

Some and some of

Before a noun with a determiner (articles, demonstratives and possessive pronouns), we use some of. If there is no determiner we use some. Some of

www.englishgrammar.org

Some says say that it is still fine to use ...

 

[5jj]

This picture is from English Grammar in Use, a reliable grammar book.

It's a popular book. That doesn't mean it's reliable.

It's suggestions are accurate up to a point, but are over-simplified.

The book says that we can use simple future- will for something that is planned.

We do not use will to convey the idea of a planned future activity. However in expressing certainty, volition or instant decision, future planned activities may be referred to.

4. the modal will + bare infinitive

The modal auxiliary verb will has a number of possible meanings. Four of the more common are:

a. certainty – John left an hour ago, so he will be home by now. (The speaker is certain that John is home.)

Context tells us whether the certainty (a) is about the present, as in the example above, or the future: Prince William will become King when King Charles dies.

b. habitual characteristic: Andrea will sit at her desk for hours without saying a word. (This is what she often does.)

Habitual characteristic (b) is something that is shown regularly. It was observed in the past, can be observed in the present, and will presumably be observed in the future. It is not possible to know of a future habitual characteristic, so we do not use this form for purely future reference.

c. volition (willingness): I’ll carry your bag for you. (The speaker is offering to carry the bag.)

Volition (c), denoting an offer, can refer only to a future situation. Will is normally contracted to ‘ll.

Fred will carry your bag for you. (The speaker is offering Fred’s services.)

d. instant decision: What will I do tomorrow? I know! I’ll go to the zoo. (The speaker decides to go to the zoo at the very moment of speaking – no plans had been made for this visit.)

Instant decisions (d) can be made only about a future situation and, usually, only reported by the person making the decision. We therefore usually encounter this meaning only with I or we as the subject. Will is normally contracted to ‘ll.



The word tomorrow in the following sentence tells us that the seeing (meeting) is in the future:

Emma will see Luke tomorrow.

Only context can tell us whether the speaker is certain of the occurrence of the future meeting, or reporting that Emma is willing to see Luke. It is not impossible that the words present an instant decision by someone wo can decide such things for other people.

See https://www.usingenglish.com/teachers/articles/ways-expressing-future-in-english.html