I am not sure I interpreted the question correctly. If I didn't please rephrase your question or ask questions.
Need can be either a verb or as a modal.
When it is used as a verb it expresses that something is to be done and it is followed by either the full infinitive:
I only need to clean the house (and then I'm done).
I need to clean only the house (and not the shed).
I need only to clean the house (and then I'm done).
The house needs cleaning.(Meaning "the house needs to be cleaned")
It is only when used as a modal that we do not have the full infinitive after "need". It is synonym of "must" or "have to" when used as a modal.
I needn't clean the house, I did it yesterday.
Need I really clean the house, can't it wait till tomorrow?
And "need" would not be used as a modal in affirmative sentences:
I have to clean the house, it's filthy.
I need to clean the house, it's filthy.
I can't go with you, I have to clean the house.
I can't go with you, I need to clean the house.