To complete the sentences appropriately using 'enough', 'not enough', or 'too', you should consider the meaning conveyed by each sentence:
Alan is old enough to vote at the elections.
'Old enough' indicates that Alan is at an age where he fulfills the minimum age requirement to vote.
This box is too heavy to carry. I need some help.
'Too heavy' suggests that the box's weight is more than what can be managed comfortably, indicating a need for assistance.
I don't like the weather in winter. It isn't warm enough .
By saying 'warm enough,' we mean that the winter weather does not meet the desired level of warmth.
My brother didn't fail the exam because he is intelligent enough .
'Intelligent enough' denotes that his level of intelligence was sufficient to pass the exam.
Do you have other pants? These ones aren't tight enough for me.
'Not tight enough' means these pants do not fit snugly, as the person prefers them.
I will live in a new house because my house is too small for six people.
'Too small' expresses that the current house's size is inadequate for accommodating six people.
My backpack isn't big enough . I can't fit all my books in it.
'Big enough' means the backpack needs to be larger to accommodate all the books.