Hello! Is this a complex grammar form... or a mistake?

I will paste the whole paragraph for the sake of context:

Let us first restate the question of how we acquire: given the correctness of the natural order hypothesis, how do we move from one stage to another? If an acquirer is at "stage 4", how can he progress to "stage 5"? More generally, how do we move from stage i, where i represents current competence, to i + 1, the next level? The input hypothesis makes the following claim: a necessary (but not sufficient) condition to move from stage to stage i + 1 is that the acquirer understand input that contains i + 1, where "understand" means that the acquirer is focused on the meaning and not the form of the message.

I would expect to have the verb conjugated with the singular form (i.e. understands). However, the words "is that" may play a role changing the grammar form, or may not.

Could you please tell me what you think about this? 

Thank you all!

Mar 7, 2017 3:05 PM
Comments · 8

Isn't it The Present Subjunctive like in these examples:

1. It is necessary that he do it immediately.

2. I demand that I be allowed to call my lawyer. => My demand is that I be allowed to call my lawyer. ?

March 7, 2017

Sentences such as "it is necessary..." introduce the subjunctive, yes.  It is simply the subjunctive form.  In the present subjunctive we do not use an "s," on the end of the third person conjugation.  For example:

"It is necessary that he see a doctor urgently."

March 7, 2017
In formal US English, the subjunctive is required in your example, so keep that in mind if you're taking a test such as the SAT or ACT. Otherwise, don't worry about it. 

Many Americans use the present subjunctive in conversation as well – I always do. My impression is that UK speakers rarely use the subjunctive in conversation, although some still use it in formal writing. 

Yes, the word "that" can be omitted.

Yes, in many cases we can avoid the subjunctive by using "for (someone) to," but this is not possible after some verbs,for example, "to suggest."
March 7, 2017

Thank you all again! now it makes sense! I checked the subjunctive out online as well and I feel it is optional to use.

I mean, can we avoid using it and say essentially the same thing? 

For example:

without subjunctive:

It was necessary for me to be there, otherwise, the students wouldn't have had any class.  

 with subjunctive:

 It was necessary (that)* I be there, otherwise, the students wouldn't have had any class.  

Usually, I would have used the 1st one, however, now with the second one I could be more emphatic. Am I right? 

*Is it ok not to use "that" in the sentence?

Thank you!

March 7, 2017
I agree with Anna and Andrew, it is Subjunctive. You can put "should" before "understand" which will make the sentence a bit easier to comprehend. 
March 7, 2017
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