Example 1

Dad will reply to the letter when he comes home from work.

Example 2

Dad will reply to the letter if he comes home from work.

The difference lies in the likeliness.

When implies high likeliness or certainty, which means the dad will definitely come home from work. If implies a lower level of certainty about that.

How about other time conjunctions? I feel like they convey high certainty. In other words, they are highly likely to happen.

Example 3

We will do the shopping before they arrive.

Example 4

As soon as he finishes collecting the data, I will start working on making the slides.

Example 5

She will go jogging after it stops raining.

Example 6

I will not go home until the construction work is done.

  • What exactly are you asking? Commented Oct 3, 2023 at 17:29
  • @PaulTanenbaum sorry for the confusion. I have edited my question. I am asking about the certainty in sentences like "You can't go out and play before you finish your homework." It seems to that it conveys that the kid will definitely finish the homework as opposed to "You can't go out and play unless you finish your homework", which sounds like the kid might not even do the homework.
    – vincentlin
    Commented Oct 3, 2023 at 17:44

1 Answer 1


"When" and "if" don't have the same function, so it doesn't make sense to compare them in terms of certainty.

A "when"-clause is a time clause, answering the question when some event will happen.

When will Dad reply to the letter?
When he gets home.

In this example, we learn when Dad will reply to the letter. It's already determined that he will, so there are no conditions to mention.

In contrast, an "if"-clause gives the conditions to be met for some event to happen, with no reference to the time of the event.

Will Dad reply to the letter?* [not "when"]
He will if he comes home.

In this example, we only know what the condition is for Dad replying to the letter, not the time. He may well come home from work, then wait a week before replying.

To demonstrate further that "if" is not a time clause, you can put both in the same question and answer without any redundancy:

Will Dad reply to the letter, and when?
Dad will reply to the letter after dinner (time) if he comes home from work (condition).

All the other conjunctions you mentioned are also time clauses, so they have the same function as "when", that is, to give the time when some event will happen with no reference to conditions.

  • Are "before" and "after" used as prepositions also the same? "He talked to my mom before buying a toy." - He bought a toy, right? "She always tells stories to her kids before going to the gym" - She definitely goes to the gym after she tells stories, right?
    – vincentlin
    Commented Oct 4, 2023 at 15:04
  • @vincentlin Yes, the rules for "before" and "after" as prepositions are the same as the conjunction forms. BUT, with "before" (in either form) it's possible that the action doesn't happen. Consider: I always talk to my wife before (I make) a big purchase. The context makes clear that it's possible I don't make the big purchase because my wife may talk me out of it. We understand from this kind of context that the sentence really means something like: "I always talk to my wife before (deciding whether to go through with making) a big purchase (that I want to make)".
    – gotube
    Commented Oct 4, 2023 at 23:19

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