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I have the following sentence:

Conventional light bulbs will be replaced with energy-saving alternatives by 2020.

According to my book, the next sentence is also correct:

Conventional light bulbs are to be replaced with energy-saving alternatives by 2020.

But I was taught that in the sentences referring to the future with by (date) we use Future Perfect.

I would write as follows:

Conventional light bulbs will have been replaced with energy-saving alternatives by 2020.

Am I right? What is the difference in meaning between these three sentences? Thanks in advance!

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    "will be replaced by X" : normal. "will have been replaced by X" : normal but the replacement time period probably doesn't include X itself, because by X the replacement is in perfect aspect. "are to be replaced by X" : someone intends to do so; the plan is to do so; but it's implied that it's not yet possible to state with absolutely certainty that it will succeed. – Luke Sawczak Jun 23 '17 at 15:13
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  1. We use "To be" for an order from a person in authority, e.g.:

The members of the jury are to report to the judge's chambers.

  1. "Will be" is just a fact that would be/occur in the future, where I am stating it as a fact, or at least a prediction, e.g.:

I will finish the assignment tomorrow.

  1. We use the Future Perfect ( "will have + pp" ) to express the idea that something will occur before another action in the future. It can also show that something will happen before a specific time in the future, e.g.:

You will have perfected your English by the time you come back from the U.S.

The Future Perfect expresses the idea that something will occur before another action in the future. It can also show that something will happen before a specific time in the future (like your third case), look more here.

  • "from a person in authority" I'm not sure that's quite right. Your example is a bit different than the OP's sentence – eques Aug 29 '17 at 14:28
  • "from a person in authority" is correct. "Conventional light bulbs are to be replaced with energy-saving alternatives by 2020." means that some governing body has mandated that conventional light bulbs be replaced. – jcarpenter2 Aug 30 '17 at 1:08
  • In context (e.g. if it only referred to the light bulbs used by a particular firm or industry) it could also mean that a directive had been laid down by company bosses, industry regulators or an advisory group, or it could be relaying the recommendation of a report that hadn't yet been adopted. But I agree with the basic point - "are to be ..." isn't so much a statement about the future itself as it is about a planned/intended future. – rjpond Aug 30 '17 at 7:43
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All three sentences are correct, and have approximately the same meaning.

The "will" future is the simplest, and is the form that most speakers would probably choose.

The second example is fairly formal in style, and indicates a plan. This is correct, as there is a plan to replace old light bulbs.

The last is also correct. The future perfect may be used to emphasise the completion of an activity in the future. As this meaning also fits the context, you may correctly use future perfect. However since the meaning of the verb "replace" already indicates completeness, the future perfect is optional. You would use to future perfect to indicate that something will happen before a certain point in time.

Although the plan was to replace old-style bulbs by 2025, government projections suggest that most will already have been replaced by 2020.

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