syntax in affecting sentence semantics

[lycen]

I am happy the next time she comes.
The next time she comes, I am happy.

The above-mentioned sentences are clear in having the same meaning.

However, I would like to know if there's a subtle difference between these (presence of auxillary verb):

I am able to do it the next time she comes. (I think "am able" here is referring to my current ability i.e. "I can do it the next time she comes." )

The next time she comes, I am able to do it. ("Am able" is referring to the future ability I am sure that I will have; zero conditional)

Thank you.

[lycen]

I was able - past
I am able - present
I will (or shall) be able - future

Isn't I can = I am able?

I can do it the next time she comes = I am able to do it next time she comes

Present tense can also be use to refer to the future, can't it?

[lycen]

I am speaks to a state of being
I am tired now
I was tired yesterday
I will be tired tomorrow

I am is used in the present. I will is used in the future.

You can't write - "I am able to do it the next time she comes" because "I am" refers to the present and the rest of the sentence refers to the future. The "I am" has to change to "I will".

What you are proposing is - I am hot next Sunday. It should be - I will be hot next Sunday.

You can write - "I can do it now" and "I am able to do it now". When the time frame changes, the sentences change, "I can do it tomorrow" and " I will be able to do it tomorrow."

I agree that "I am hot next Sunday" is unacceptable. However, for scheduled
future events for instance "I start my new job next Monday" and "My train leaves on Saturday" are as correct as "I'll start my new job next Monday" or "My train will leave on Saturday", aren't they? I got them from a Cambridge grammar book.

[philo2009]

I agree that "I am hot next Sunday" is unacceptable. However, for scheduled
future events for instance "I start my new job next Monday" and "My train leaves on Saturday" are as correct as "I'll start my new job next Monday" or "My train will leave on Saturday", aren't they? I got them from a Cambridge grammar book.

You are right that this is possible in principle. The event, however, must be one naturally capable of being 'scheduled' in some way. An ability, or indeed any 'state of being' in the future (as opposed to an action such as 'leaving' or 'arriving'), can hardly be so considered, hence the rarity of ever finding the verb 'be' used in this way.

'Can' (but normally not 'be able to') is an exception to this general rule, since we may refer by means of it to an expected future ability to accomplish some particular task at a certain future time, but normally only where this constitutes an undertaking/promise of some kind, e.g.

I can see you briefly next Thursday morning.

but not

*By the end of the course, I can speak French fluently.

(-->...I'll be able to...)

[philo2009]

I am happy the next time she comes.
The next time she comes, I am happy.

The above-mentioned sentences are clear in having the same meaning.

However, I would like to know if there's a subtle difference between these (presence of auxillary verb):

I am able to do it the next time she comes. (I think "am able" here is referring to my current ability i.e. "I can do it the next time she comes." )

The next time she comes, I am able to do it. ("Am able" is referring to the future ability I am sure that I will have; zero conditional)

Thank you.

It is difficult to answer this question, since both sentences are unidiomatic. If, however, we change 'am able to' to 'can', then we will get

[1] I can do it (the) next time she comes.

and

[1a] (The) next time she comes, I can do it.

which would, in fact, be identical, both structurally and semantically, differing only in the obvious, minor respect of clause-ordering, with [1] being slightly the more natural choice.

'Can' here would naturally be understood in either as having a future meaning (please refer also to my comments on this point in post #6).

Note, incidentally, that the term 'conditional' applies only to sentences containing, or implying, 'if'. It is therefore not applicable here.

[philo2009]

Yes - If you change "I am" to "I can" then things change. The point is that "I am" cannot be used when referring to the future.
As I have already indicated. The rephrasing with 'can' was offered simply as a correct example of the type of sentence that the questioner was apparently endeavouring to make (i.e. one referring to future ability).

I'm not sure if I understand your comment on the conditional being restricted to "if" statements -
I was referring to the application of the term conditional to a sentence-type. Conditional sentences are those consisting (typically) of a main clause modified by an adverbial if-clause. They do not necessarily contain forms of the conditional mood (i.e. [would V] etc.).

"They could have went to the moon or to Mars."
I believe you mean 'could have gone...'.

[lycen]

It is difficult to answer this question, since both sentences are unidiomatic. If, however, we change 'am able to' to 'can', then we will get

[1] I can do it (the) next time she comes.

and

[1a] (The) next time she comes, I can do it.

which would, in fact, be identical, both structurally and semantically, differing only in the obvious, minor respect of clause-ordering, with [1] being slightly the more natural choice.

'Can' here would naturally be understood in either as having a future meaning (please refer also to my comments on this point in post #6).

Note, incidentally, that the term 'conditional' applies only to sentences containing, or implying, 'if'. It is therefore not applicable here.

Isn't "I can do it the next time she comes" akin to "I can do it when she comes"? Isn't "when" also part of the conditionals just like "if" as shown here: ENGLISH PAGE - Conditional Tutorial ?

What about talking about the general future (changing "next time" to "if"):

I can(am able to) do it if she comes.
If she comes, I can(am able to) do it.

[lycen]

I will start my new job next Monday.
I start my new job next Monday - This is a case of elliptical construction because a word is missing that is required by the other elements. An example of this form is, "Shoot when ready." The "you are" is understood (Shoot when you are ready).

The debate is really about "I am". Your contention is that, “I am able to do something tomorrow.” is correct. It is not (elliptical, huh?).

What's written in my Cambridge Grammar book: You can say that somebody is able to do something, but can is more usual:

Tom can come tomorrow
Tom might be able to come tomorrow
and I don't see why "Tom is able to come tomorrow" is wrong.

I will start my new job...- future form
My train will leave...- future form

I can't discern a difference between can & be able to

[Tdol]

'Am able to' has a similar meaning to 'can' and in many cases they can be used interchangeably, but that does not mean that they have to follow exactly the same behaviour grammatically. Can is a modal and be is not, just as must and have to can have similar meanings, but they don't always behave exactly the same way. A synonym of an irregular verb doesn't have to be irregular and a synonym (or near synonym) of a modal isn't a modal.

There are contexts where I think 'I am able to do it tomorrow' might work- if you trying to make an appointment and I have my diary open in front of me and see that I am free tomorrow, then I don't think it would be an error to use it because I could look at it as a current ability. However, this is stretching a point and I would be far more likely to say 'I can do it tomorrow'. Things like 'the next time she comes, I am able to do it' might be heard in speech, but it's pug ugly.

Isn't I can = I am able?
Not in every way, no

[lycen]

'Am able to' has a similar meaning to 'can' and in many cases they can be used interchangeably, but that does not mean that they have to follow exactly the same behaviour grammatically. Can is a modal and be is not, just as must and have to can have similar meanings, but they don't always behave exactly the same way. A synonym of an irregular verb doesn't have to be irregular and a synonym (or near synonym) of a modal isn't a modal.

There are contexts where I think 'I am able to do it tomorrow' might work- if you trying to make an appointment and I have my diary open in front of me and see that I am free tomorrow, then I don't think it would be an error to use it because I could look at it as a current ability. However, this is stretching a point and I would be far more likely to say 'I can do it tomorrow'. Things like 'the next time she comes, I am able to do it' might be heard in speech, but it's pug ugly.

Isn't I can = I am able?
Not in every way, no

What I understood from the above, "am able to do" might not be grammatically wrong in my example but just horrible to hear/ not often used. In that case, Gillnetter assertion that it's incorrect just doesn't cut it.