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Read about auxiliary verbs here.
One unique feature of auxiliary verbs is that they function as both auxiliary and lexical verbs. The auxiliary function is the primary function of all auxiliary verbs, but they sometimes go beyond their limit by functioning as lexical/main verbs.
Examples:
1. I was reading my book.
Aux. Lex.
2. I was there!
Lex.
In sentence one, the auxiliary verb 'was' is functioning as an auxiliary/helping verb(which is its primary function). You can see it helping the lexical verb, 'reading' in the sentence, which shows that it is syntactically dependent. However, in sentence two, it is functioning as a lexical verb. It is neither dependent to nor helping any lexical verb in the sentence.
On the other hand, not all auxiliary verbs function as auxiliary verbs. The auxiliary verbs, 'doing' and 'done' only function as lexical verbs. These two auxiliaries chose to only function as lexical verbs perhaps because they are very stubborn. Lol!
Now let's see some examples where the auxiliary verb, 'done' is functioning as a lexical verb:
1. He has done the work.
A. L.
2. They have done the work.
A. L
3. The deed has been done.
A. A. L
In the above sentences, you can see the auxiliary verb,'done' functioning as a lexical verb. You can never see this auxiliary(done) performing the function of the other auxiliaries('have', 'has', and 'been') in the sentences above.
Now let's see some examples of the auxiliary, 'doing' functioning as a lexical verb:
1. I am doing my assignment.
A. L
2. She is fond of doing nasty things.
L
3. You were doing your assignment when I came in.
A. L
4. She is doing her kitchen chores.
A. L
In each of the sentences above, the auxiliary verb, 'doing' functions as a lexical verb.
In sum, auxiliary verbs are unique as they can perform both the auxiliary and lexical functions of a verb. However, the primary auxiliaries, 'doing' and 'done' don't buy into this 'double function' of other auxiliary verbs because they only function as lexical verbs despite the fact that they are part of the 'auxiliary' family.