Which of the following sentences is correct?
When we know what she decides, we will see what to do.
When we will know what she decides, we see what to do.
When we will know what she decides, we will see what to do.
The reason I am asking is that in Italian I would use the future tense for both the clauses, and I would tend to do the same in English too.
SHORT ANSWER:
Only your first sentence is idiomatic English.
LONG ANSWER:
First, will is employed in when clauses only if when is employed as a relative adverb, not if when is employed as a subordinating conjunction:
You will arrive in Milan at 4 pm, when you will be met by our representative.
✲When you will arrive in Milan you will be met by our representative.
Second, the ‘sequence of tenses’ rules for sentences with [when] ... [then] and [if] ...[then] constructions are very similar, so I’ll treat them together.
NOTES: I bracket [when], [if] and [then] because these words may only be implied and not actually appear in a sentence. Also, I have changed the words in your example to permit what I hope is a more transparent explanation.
The rules vary depending on whether the sentence expresses habitual practice or, as in your example, a single contingency.
If the sentence expresses a repeated or habitual practice, both clauses will have the same tense: either simple present or simple past. In these cases, [when] or [if] bears the sense whenever.
[Our practice is: ] Whenever we see this, we do that.
[Our practice was:] We did that whenever we saw this.
When the sentence deals with a single occasion, English employs a sequential structure, in which the [then] clause is depicted as ‘following’ the [when] or [if] clause. In practice this means that
In sentences with present reference, a simple present or present perfect in the [when/if] clause is paired with a will + unmarked infinitive construction in the [then] clause.
NOTE: 1) The order of the clauses in the sentence is irrelevant, and 2) I speak of a ‘construction with will’ rather than of a ‘future tense’ because the will signifies consequence quite as much as it does futurity.
[We have decided:] If we see this, we will do that.
[We have decided:] We will do that when we have seen this.
Sentences with past reference are ‘backshifted’: a simple past or past perfect in the [when/if] clause is paired with a would + unmarked infinitive construction in the [then] clause.
[We decided yesterday:] If we saw this, we would do that.
[We decided yesterday:] We would do that when we had seen this.
To decide which sentence is correct, put them in normal word order, which means "Begin the sentence with the independent clause instead of the dependent clause":
We will see what to do when we know what she decides.
We see what to do when we will know what she decides.
We will see what to do when we will know what she decides.
It seems clear to me that only the first comes close to being idiomatic English. I'd revise it to read thus:
We {will / shall} see what to do {when / after} we know what she has decided.
Italian and English apparently don't use the same sentence structure rules. The sequence of tenses is different in these languages.
