I wonder if we join two sentences with the same conjunctions (if, when, because, etc.), can we use the conjunction only once in the combined sentence?

For example, if I combine the two sentences below:

A.We will go out if it does not rain. + We will go out if our father allows us.

A.1. We will go out if it does not rain and if our father allows us.

A.2. We will go out if it does not rain and our father allows us. (omitting if)

Which sentence, A1 or A2, is correct or preferable?

Another example is:

B.The game will resume when it stops raining.+ The game will resume when all competitors are ready.

B.1. The game will resume when it stops raining and when all competitors are ready.

B.2. The game will resume when it stops raining and all competitors are ready. (omitting when)

Which sentence, B1 or B2, is correct or preferable?

I have tried checking several online grammar websites and searching here but found no answers. I think it must have been asked before, but I just couldn’t find it probably because I didn’t know how to ask the question correctly. So, if that’s the case, please just provide me the link to the answer; I’d be very grateful.

  • 2
    The rule that allows deletion of repeated words (articles, pronouns, prepositions, etc) in the second conjunct of compound phrases or clauses held together by and, or, but is called Conjunction Reduction and is very, very common. Mar 8 at 18:39
  • @JohnLawler Thank you very much. I wonder if conjunction reduction is acceptable in formal writing such as research report and academic article.
    – user287279
    Mar 10 at 1:00
  • Oh, yes, it's necessary. Actual research reports and academic articles are rarely "formal"; they're more about being specific and avoiding ambiguity. There is no "incorrect" problem; it's always about being clear and precise. Mar 10 at 17:20
  • @JohnLawler Thanks.
    – user287279
    Mar 12 at 14:47

3 Answers 3


In English - as is the case with other languages - there are often multiple ways of saying essentially the same thing. However, each phrasing tends to project its own nuance, or flavour, that might be missing, muted, or conversely, heightened, when phrased differently.

In conditional of the form "If X then Y", X doesn't have to be a simple proposition such as "it is raining" - it can be a compound proposition like "it is raining and we have your father's permission". That is, grammatically, you're allowed to 'logically' combine multiple propositions under a single 'if'.

Let's consider your first pair of propositions:

  • it isn't raining
  • your father permits it

If you wanted to simply express a clinical conjunction of the two conditions, you can place them under a single 'if':

  • If it isn't raining and your father permits it, then ....

But suppose the weather is merely a pragmatic condition, while gaining permission is a rather more serious affair. Then you might pull the second condition out, placing spoken stress on the second "if":

  • If it isn't raining and if your father permits it, then ....

You can emphasise it even more by replacing the second 'if' with "only if", and so on. You can even retain the original compound proposition and say the "and" more loudly and stretch it out for a longer duration to emphasise the second condition.

Each of these constructions is grammatical and logically interchangeable. But there can be subtle (or emphatic) differences that push you to picking one phrasing over another.

  • 1
    This answer makes the important point, which may be crucial for the OP's purposes, that adding the second if need not be pragmatically superfluous, even when it is semantically superfluous. I, however, cannot resist making the pedantic observation that the answer treats the OP's examples of the 'Y, if X' form as interchangeable with 'if X, then Y', which they may not always be. 'If X, then Y', is standardly treated as meaning that X is a sufficient condition for Y, while in the OP's examples, the absence of rain and the permission seem to have been intended as necessary conditions.
    – jsw29
    Mar 8 at 17:30
  • 3
    Repeating "if" as well as providing emphasis may have the advantage of being clearer in some circumstances, e.g. if there is negation or auxiliary verbs and it's not clear what their scope is, or some other complexity to the condition, then repeating "if" provides structure.
    – Stuart F
    Mar 8 at 17:34
  • @jsw29: That’s not just pedantic, it’s misleading. The “if … then …” conditional of everyday English (and other natural languages) is not the material conditional of formal logic, or even of mathematical English. In many contexts, the meaning of “if X then Y” is just as much about necessity as sufficiency, as OP’s examples well illustrate.
    – PLL
    Mar 8 at 22:18
  • 1
    Also, if you have a lengthy list of propositions, repeating the "if" ("If A, if B, if C, if D, if E, and if F, then G") gives a more pessimistic connotation ("There's no way we're going to meet all of these conditions").
    – Dan
    Mar 8 at 22:33
  • 2
    @PLL: I'm now imagining a future where after teaching my children (currently 2 and 5 years old) the ways of logic, I tell them, "If you finish your vegetables, you can eat your dessert", and they immediately eat their dessert arguing, "You didn't say 'if and only if'!" Mar 9 at 15:06

A.2. We will go out if it does not rain and our father allows us. (omitting if)

B.2. The game will resume when it stops raining and all competitors are ready.

In essence, the arguments in the above can be seen as enumerated as in a list:

We will go out if

(i) it does not rain


(ii) our father allows us.

If has not been omitted - it simply applies to both conditions.

  • Thank you. I think so too. But as I'm not a native speaker, I wasn't sure whether it's grammatically correct. So, is this applicable to the case of "that" too? ... "He thinks that A is correct." and "He thinks that B is wrong" can be combined into "He thinks that A is correct but B is wrong.", can't they?
    – user287279
    Mar 8 at 13:45
  • Btw, from what I've searched, this way of writing seems like "conjunction reduction"? Is it so?
    – user287279
    Mar 8 at 13:48
  • @user287279 (i) Yes or even "He thinks that A is correct but/and B wrong."; (ii) Yes.
    – Greybeard
    Mar 8 at 16:30

The use of “when” in the second part is equivalent to “if”, in the sense of “in the circumstances that”. I therefore only consider the use of “if”.

Logically, when A and B are conditions that can be true or false,

“A and B” is only true when both A and B are true.

If “A and B” is only true when when both A and B are true

“If A” and “If B” is only true when both A and B are true.

Therefore, when A is true and B is true, the two statements equate:

If “A and B” = “If A” and “If B”

Similar argument shows that if A or B or both are false, the two versions are false.

Consequently, your four versions are logically the same, which suggests that all four statements are grammatically acceptable.

  • 1
    Thank you very much. But I think my question must be confusing, A and B are unrelated examples; they are just separated examples. The question was intended to ask whether A1 or A2 is correct and whether B1 or B2 is correct. ... I'll edit my question to make it clearer. Thanks again for the help.
    – user287279
    Mar 8 at 9:08
  • I have added my reasoning regarding the equivalence of if and when. I hope this helps.
    – Anton
    Mar 8 at 9:51

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