The class of marginal auxiliary verbs has somewhat fuzzier edges than the other auxiliary verbs, and different grammatical approaches use the term in different ways. Here, we take a narrow view and include only the verbs dare and need, together with ought to and used to.
There are two properties that unite this narrow class of marginal auxiliaries. First, in terms of meaning, they resemble modal auxiliaries like can, must, etc. Second, they can be constructed either as pure auxiliaries or as main verbs without a difference in meaning. Thus, they differ from primary auxiliary verbs, which also occur in different constructions, but with a clear difference in meaning between the auxiliary and main verb uses.
Dare and need
Both dare and need can be used as auxiliary verbs. In this use, like other auxiliaries, they precede not and they invert with the subject in the formation of yes/no questions.
(1) I dare not speak to the Dean about this. (cf. the modal auxiliary would: I would not speak to the Dean...)
(2) We need not tell the Dean about this. (cf. the modal must: We must not tell the Dean about this.)
(3) Dare I speak to the Dean about this?
(4) Need we tell the Dean about this?
Both verbs may also be constructed as main verbs. In this use, like main verbs, they require do-insertion when negated by not and in yes/no questions:
(5) I did not dare to speak to the Dean about this.
(6) We do not need to tell the Dean about this.
(7) Do I dare to speak to the Dean about this?
(8) Do I need to tell the Dean about this?
The main verb construction is also distinguished by the form of the following verb. Thus, whereas auxiliaries are followed by the so called bare infinitive, which lacks the infinitive marker to, in the main verb construction, dare and need are followed by the to-infinitive.
Moreover, when used as as main verbs, both dare and need agree with the verb, e.g. by taking the 3rd person singular -s. The first pair of examples below illustrates the auxiliary construction, whereas the second pair illustrates the main verb construction:
(9) No one dare approach him before breakfast.
(10) Everyone need approach him at the right time.
(11) No one dares to approach him before breakfast.
(12) Everyone needs to approach him at the right time.
The main verb construction is the most common alternative in positive declaratives, as in our last examples. However, the auxiliary construction has some currency in academic texts, especially with need.
Ought to and used to
The reason for giving the forms as ought to and used to (instead of ought and used) is simply that they only very rarely occur without to. Thus, from this perspective they behave as main verbs. However, with respect to their behaviour in negative clauses and in yes/no questions, they often behave as auxiliaries, as in the following examples:
(13) We ought not to tell the Dean about this.
(14) Ought we to tell the Dean about this?
(15) We used not to prepare the samples as carefully as this.
With used to, the only form of yes/no question in current usage is the main verb construction (with do-insertion), as in (17). In clauses negated by not, such as (16), the main verb construction is also common:
(16) We did not use to prepare the samples as carefully as this.
(17) Did you use to prepare the sample as carefully as this?