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What is the grammatical and correct structure for "if"?

For example which of the following structures are valid?

If they are black, then they should be selected.

vs.

If they are black, they should be selected. 

vs.

If they are black, should be selected.

Is specially the last one correct? or has any application?
If not, how can I avoid the repetition of subject in clauses?

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  • @Araucaria, Thank you for your advice, I cleared the tick form the accepted answer, if you like please post your answer.
    – Ahmad
    Commented Jun 8, 2015 at 15:19
  • Btw, my last comment had a typo in it. In English you can usually drop identical subject in the subordinate clause, but not the main one. Commented Jun 8, 2015 at 15:34
  • I might try to write you an answer later if I get time (am just off to teach a lesson right now). But I think @Sanchises is going to edit their post to add that extra info in :-) Commented Jun 8, 2015 at 15:35

1 Answer 1

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The first two are both correct. The first version is stronger than the second - one would likely put extra emphasis on the then. To the native English speaker, this will mean something similar to 'only then should they be selected'.

The third sentence is incorrect, since the second part does not have a subject. Since an if/then construction relates two complete sentences, both should usually (see below) have a subject and main verb. For example, one could also say 'If the hamburgers are black, the barbecue should be left to cool' - two completely different subjects! So, one cannot assume the subject to be the same in both parts of the sentence.

As Araucaria mentioned, there is a possibility to drop the repetition by using the construct if [property of subject], [main clause]. For example, if black, they should be selected. It's basically a shorter version of 'If the subject is [property of subject], [main clause]'. Example: 'If the button is red, the button should be pushed' could be shortened to 'If red, the button should be pushed'. Care should be taken not to say things like 'If red, you should push the button' - then you're saying the person pushing the button should be red, not the button!

An alternative construct without repetition would be 'Those that are black, should be selected'.

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  • Erm, that's not quite correct. Only the main verb needs to have a subject explicitly expressed. If the subject of the main clause and the subordinate clause are the same, the subject of the subordinate one can be dropped - the verb will need to be changed so that it is no longer tensed If coming on Thursday, he always brings a box of chocolates with thim. Commented Jun 8, 2015 at 15:14
  • @Araucaria True, I forgot to consider the case you mention in the comment above (If black, they should be selected). I'll try and edit that in.
    – Sanchises
    Commented Jun 8, 2015 at 15:31
  • @Araucaria Better? ;) Thanks for the feedback.
    – Sanchises
    Commented Jun 8, 2015 at 15:39
  • Thank you much, but what about more complicated sentences, for example If coming on Thursday, he always brings a box of chocolates with thim, those which has verbs... what is the structure? could you please give some examples?
    – Ahmad
    Commented Jun 8, 2015 at 15:45
  • Yes, better, but the second paragraphs not quite right because it implies that you can't drop the subject in the subordinate clause even if they're the same. :) Commented Jun 8, 2015 at 15:45

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