For the idiom "We will cross that bridge when we come to it", shouldn't it be "We will cross that bridge when we will come to it"? If not, please help me understand how the latter is grammatically incorrect.
In "We will cross that bridge when we come to it", "come" is the simple present tense used to indicate the present in the future.
It is very common:
"I am in Paris tomorrow - I'll speak to you then"
"John leaves in an hour, don't let him forget his coat."
In "when we come to it", when = at the time that, i.e. the present tense is used to create the present at that time.
The simple answer is that usually, the word "will" isn't used after "if", "when", or similar words.
Incorrect: When the sun will rise, I will get out of bed.
Correct: When the sun rises, I will get out of bed.
Incorrect: If you will see David tomorrow, say hello to him for me.
Correct: If you see David tomorrow, say hello to him for me.
Incorrect: After we will eat dinner, we will have dessert.
Correct: After we eat dinner, we will have dessert.
Incorrect: If Sarah will cross the finish line first, she will win the race.
Correct: If Sarah crosses the finish line first, she will win the race.
Incorrect: We will cross that bridge when we will come to it.
Correct: We will cross that bridge when we come to it.
The idiom in the title of the question is correct.
From Merriam-Webster's definition of cross that bridge when one comes to it:
: to not worry about a possible problem until it actually happens
// I don't know how we'll pay the bills if you quit your job, but we'll cross that bridge when we come to it.
Consider the following variations with only when or will:
✔ When we come to that at some point, we will cross it.
✔ We will come to that bridge at some point.
And also the following variations with both words:
✔ We don't know when we will come to that bridge.
✔ When we will come to that bridge is unknown.
✘ When we will come to that bridge at some point, we will cross it.
It's this particular combination of when and will that is unnatural.
It seems that using when with will only forms an acceptable sentence if it's then followed by just a verbal phrase or just a verb and an adjective.
But using the combination in a dependent clause that introduces an independent clause is not idiomatic. Unfortunately, I don't know exactly why this would be, and I can't put the reason behind it into an actual rule.
However, I do know which version of the sentence is idiomatic and which isn't:
✔ We will cross that bridge when we come to it.
✘ We will cross that bridge when we will come to it.
As we can see in the Cambridge Dictionary's grammar section
The present simple is used to refer to events in the future which are certain because they are facts, or because there is a clear or fixed schedule or timetable
with examples like
- I work tomorrow
- What time does their flight to Seoul leave?
The second example can be changed to "When does the flight to Seoul leave?" without a change in meaning. You can imagine the answer being something like "When the airplane is ready", not "When the airplane will be ready".
"When the time comes" is a similar usage of the present simple to refer to something in the future.