subject auxiliary inversion - 'Only by defending against threats WILL YOU be able to carry out your strategies' - Position of "will"

I saw this sentence and thought shouldn't "will" be placed after "You" instead of before it?

Only by defending against your opponent's threats will you be able to successfully carry out your own strategies.

Answer

Short answer

Sometimes only means 'not - except in this situation'. If we use it to modify an Adjunct at the beginning of a sentence, we need to change the order of the Subject and the auxiliary verb in the main clause. We need the same word order that we find in questions:

• [Only in extreme circumstances] will I steal.

If we don't do this the sentence is ungrammatical:

• *Only in extreme circumstances I will steal. (not good)

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Full Answer

An Adjunct is a word or phrase that gives you extra information in a clause or sentence. It isn't a Subject or a Complement of the verb (for example an Object). An Adjunct is not essential for the grammar. It is extra information that we usually put at the end of the sentence in English. Adjuncts can be preposition phrases, adverb phrases or noun phrases. In these examples the Adjuncts are in bold letters:

• Maria will offer to help when she understands that the situation is very serious.


• I found out what happened by chance.


• I noticed Pavarotti after the concert



• They will steal things if they are starving.


• My girlfriend speaks to me after I have had my first coffe in the morning.


• They told me the news recently.

Notice that if we want to we can put the Adjuncts at the beginning of the sentence:

• When she understands that the situation is very serious, Maria will offer to help .


• By chance I found out what happened .


• After the concert I noticed Pavarotti


• If they are starving they will steal things .



• After I have had my first coffee in the morning my girlfriend speaks to me.


• Recently they told me the news.

Now, if we want to say that the thing in the main clause only happens in the situation described in the Adjunct, we can use the word only. The normal place to put this word is after the first auxiliary verb. If there is no auxiliary verb, we put it before the main verb:

• Maria will only offer to help when she understands that the situation is very serious.


• I only found out what happened by chance.


• I only noticed Pavarotti after the concert


• They will only steal things if they are starving.


• My girlfriend only speaks to me after I have had my first coffee in the morning.



• They only told me the news recently.

Now suppose we want to move this adjunct to the front of the sentence. Let's try:

• When Maria understands that the situation is very serious, she will only offer to help .

This sentence is grammatical, but it means something very different. The problem is that the word only nearly always emphasises something in front of it. It can't normally emphasis something earlier in the sentence. The first sentence means that she won't do anything else, she will only offer to help. We wanted it so say that she will offer to help but only when she understands about the serious situation. The second sentence means they only told me the news, they didn't do anything else. We need it to say they told me the news, but only recently and not before.

So, we can fix this if we put the word only directly before the Adjunct. So for example we can write Only when she understands the situation .... Let's try it:

• *Only when Maria understands that the situation is very serious, she will offer to help. (ungrammatical)

Ooops. That's not a good result. This sentence is ungrammatical. The word only has several meanings. We are interested in the one which shows that something doesn't happen except in this situation. When we put this only before an Adjunct at the beginning of a sentence, we need to change grammar in the main clause. We need to invert the Subject and the auxiliary verb. We need to put the auxiliary before the verb:

• [Only when Maria understands that the situation is very serious], will she offer to help.

If the sentence was in the present simple or past simple, there will not be an auxiliary, we will need to use the dummy auxiliary DO, like we do in negatives and questions:

• [Only by chance] did I find out what happened.

Here are the other examples, this time with [only + Adjunct] at the beginning:

• Only after the concert did I notice Pavarotti.


• Only if they are starving will they steal things .


• Only after I have had my first coffee in the morning does my girlfriend speak to me.


• Only recently did they tell me the news.

The Original Poster's example:

Here only comes before an Adjunct at the beginning of the sentence. We need subject-auxiliary inversion here. The will needs to come before the you:

[Only by defending against your opponent's threats] will you be able to successfully carry out your own strategies.