In English, an Auxiliary verb is a verb that changes or helps another verb. Auxiliary means "extra" or "different". Auxiliary verbs like do can be used to make a question. These verbs are usually used with other verbs.
Basic examples
Below are some sentences that use auxiliary verbs from English, Spanish, German, and French, with the auxiliary verb in bold:
a. Do you want tea? – do is an auxiliary verb that changes the main verb want to make a question – see do-support.
b. He has given his all. – has is an auxiliary used to say the perfect form of give.
c. He cogido tu lápiz. – he is an auxiliary accompanying the main verb coger, used here to make a compound verb, the perfect present of the verb coger.
(I) have grabbed your pencil = 'I have taken your pencil.'
d. Das wurde mehrmals gesagt. – wurde 'became' is an auxiliary used to make the passive voice in German.
That became many times said = 'That was said many times.'
e. Sie ist nach Hause gegangen. – ist 'is' is an auxiliary used with movement verbs to make the perfect tense in German.
She is to home gone = 'She went home/She has gone home.'
f. J'ai vu le soleil. – ai 'have' is an auxiliary verb used to make the perfect tense/aspect in French.
I have seen the sun = 'I have seen the sun/I saw the sun.'
f. Nous sommes hébergés par un ami. – sommes 'are' is an auxiliary verb used to make the passive voice in French.
We are hosted by a friend.
These auxiliaries help make a question, show tense, or make passive voice. Auxiliaries like these usually appear with a normal verb that carries the main meaning of that part of the sentence.