An ELT Glossary : Assertive and Non-Assertive Meaning

Assertiveness regards the speaker/writer's view of the validiity of the proposition being expressed. If the speaker regards what they are saying as true, factual, existing etc, they will express it assertively:
John is here.
I'm sure John's here.

On the other hand, if they are not sure of the validity of the proposition - if they wish to negate or query it, they will express themselves non-assertively :

John isn't here.
I doubt if John is here.
I wonder if John is here.
Is John here?

This is important in English as many words are restricted to either an assertive or non-assertive context. Fine for example is always used assertively, while far is always used in non-assertive contexts. (Note that an asterisk indicates that the sentence is incorrect English)

He's fine.
I'm sure he's fine.
* I'm not fine today.
*I doubt if they're fine.
* I wonder if he's fine
*Are you fine?

* It's far.
* Undoubtedly, it's far.
It's not far.
I don't think it's far.
I wonder if it's far.
Is it far?

Words which are restricted in this way will usually have a "partner expression, which can be used either in the opposite context or in both contexts. For example : Fine is "partnered" by well and far by  a long way :

* It's far  but   It's a long way. 
* I'm sure it's far  but  I'm sure it's a long way.
It isn't far / It isn't a long way.
I doubt if it's far / I doubt if it's a long way
I wonder if it's far / I wonder if it's a long way
Is it far? / Is it a long way?

Other examples of  "partner" expressions include :

already (assertive)  /  yet (non-assertive)
some (assertive)  / any (non- assertive)  and related words something/anything, someone/anyone etc
a little (assertive)  / little (non-assertive)
a few (assertive)  / few (non-assertive)
still (assertive) / any longer - any more  (non-assertive)
too and  as well (assertive) / either  (non-assertive)
Much/many (non-assertive)  / a lot of  (all contexts)

The last example needs further clarification, however. The restriction to non-assertive contexts of much and many is true of neutral-informal English when  much/many are in object position:

He has a lot of friends /money    not  *He has many friends / much money
I don't think he has many friends/much money 
Does he have many friends /much money?

But in subject position  or as complement  much and many are often used assertively :

Many people have said that....
There has been much talk about....

Semi modal verbs are also affected by assertive/non-assertive contexts. When used in non assertive contexts, either the lexical form or the modal form can be used.
You don't need to worry.
You needn't worry.

But in assertive contexts, only the lexical form is possible.
I need to talk to John
*I need talk to John