the difference with "if" sentences with "will"

Question

"I'll order from you if It will be in stock"

Gotube gave me this example here to show me that "will" was possible with if sentences, he said that in this case the condition was not a really future event and the rule of not using "will" did not apply there.

He then gave me another example "we'll move the party inside if it rains" where you couldn't use "will" because here the "will" described a really future event.

But when you say "We'll move the party inside if it rains" it is also because there is a present evidence of future state. May be there are some dark clouds in the sky or may be you've heard the weather report so the condition is not a future event like in the first example.
I am completly lost I don't understand the difference gotube tried to explain me.

Answer 1

The difference I was trying to explain in that other question (link now added to question above) is between the condition happening in the future, or the condition being the knowledge (now) of a future event.

Let's compare these two sentences:

1. I'll move the party indoors if it is going to rain.

2. I'll move the party indoors if it rains.

In the first, the decision to move the party indoors will happen when we get a report that it is going to rain, like from our weather app or the news. For instance, if someone at that moment confirmed that it was going to rain, the plan for the party would change at that moment.

In the second, the decision to move the party indoors won't happen until rain drops begin to fall during the party. Sentence 2 cannot describe a situation where the party will move inside if a weather report says it's going to rain in the future.

Answer 2

OP's question turns on the fact that in the "order" example, speaker is saying I will place my order right now.

It doesn't work for me with, say, I'll order from you tomorrow if it will be in stock then. In that slightly shifted context, it would have to be ...if it is in stock then

I assume that in the "party" example, nobody would think speaker is suggesting they should take steps right now to relocate the future party simply because someone has just told them it will rain tomorrow.

The general idea here is that we only explicitly indicate true "future tense" once in any given utterance. But I'll in the "order" example doesn't really reference "future" - it's more of a "currently conditional consequence" usage.