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I got puzzled when putting the following pieces together. I checked it online but couldn't find a convincing explanation.

In large supermarkets, management must decide (on) what to put on sale.

Is the "on" preposition necessary here? What difference would it make if we removed it?

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The main difference if you remove it is that your sentence won't have the totally unnecessary / clunky repetition of the word "on". Well, to be honest, that's the only difference. – FumbleFingers Mar 31 '12 at 20:53

You have cited an extraneous preposition, i.e., one that could be removed from a sentence without any loss or change of meaning. Generally speaking, it's best to omit them altogether. Here are a few more examples:

We need to clean (up) the kitchen.
He's standing inside (of) the fence.
I saw him jump off (of) the bed.

Sometimes, superfluous prepositions are found at the end of sentences or questions; these should be omitted:

Where are they (at)?
Where are they going (to)?

For the non-native speaker, though, such prepositions can be hard to detect, because some are necessary, or idiomatically correct. For example, in the following sentences, the prepositions in parentheses should not be removed:

He agreed (to) meet me at the cafe.
The song of the robin differs (from) the song of the sparrow.
They will show (up) sometime after lunch.
Suddenly, it dawned (on) him: "I forgot my keys!"

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I believe that the preposition in, "They will show (up) sometime after lunch," is extraneous. The "up" is quite commonly removed from the sentence with no loss in meaning. – user1873 Mar 31 '12 at 23:34
@user1873: Yes, it can be removed, but that's technically informal usage – show up is defined as to come to or arrive at a place. See definitions 37c and 17 at Dictionary.com. Show up shows up as a dictionary entry all on its own; ergo, the up is more idiomatic than extraneous. – J.R. Apr 1 '12 at 0:41
+1 for deprecating superfluous words. – htoip Apr 7 '12 at 0:47
@J.R., Is there such a thing as "I saw him jump off of the bed"? – Pacerier Nov 17 '15 at 23:09
@user1873,"They will show sometime after lunch" sounds... odd. – Pacerier Nov 17 '15 at 23:10

In your example, the preposition on is not needed, as others have pointed out. The meaning is not changed by deleteing it, so I agree...delete it. I think "decide on what" is considered poor grammar by many, but it is commonly used. (Just Google "decide on what" in quotation marks and you get plenty of hits.)

Note that if you have a sentence with a single object (instead of a phrase that begins with a determiner such as "what to put on sale" or "which question to answer"), the preposition is needed.

As an example: The bride must decide on a dress. (You wouldn't write or say: The bride must decide a dress.)

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+1 for the note on determiners. – chrisdillon Apr 1 '12 at 2:36
an anonymous user comments: EDIT: The explanation above is not entirely correct. In the example given: "The bride must decide on a dress." [a dress] is a noun and the object of [decide on]. No doubt, the sentence is correct. However, you can also say "The bride must decide on [which dress to wear] or [what to wear]. These two are noun phrases and therefore can occupy the same syntactic postion as [a dress]. So, you can use both a simple object or a phrase acting as an object so long as the syntactic structure of "decide on + N/NP" is fulfilled. – StoneyB Feb 9 '13 at 2:47
@StoneyB, not sure if this was your edit (a little confusing the dates, and note about an anonymous user commenting), but I disagreed with it. I would not use "on" in either of those examples. I would say, "The bride must decide which dress to wear." or "The bride must decide what to wear." So I rolled back the edits. – JLG Feb 9 '13 at 19:43
@JLG I reviewed the edit, and I thought I rejected it. My contribution consisted of copying that long anonymous comment ( probably put in because the visitor had insufficient rep) and adding it above. At any rate, now I'm confident the answer I'm looking at is in fact the answer, +1. – StoneyB Feb 9 '13 at 19:49
@JLG, And why is "decide on what" is considered to be "poor grammar"? – Pacerier Nov 17 '15 at 23:09

Use the preposition "on" to imply an unstated object:

The Supreme Court will decide on the health-care law soon.

(the decision is a separate entity and not merely selecting from a finite set of options)

or before a gerund (verb as a noun):

Odisha to decide on rescuing hostages.

or before an object's modifiers:

NOAH to Decide on New Food Pantry Location.

In your example, I would go without the preposition because the object directly proceeds the verb.

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