Final answer:
The last two letters of the infinitive form of any English verb are 'to'. Auxiliaries like 'could', 'must', and 'ought to' are part of the verb phrase and are not separate adverbs. They modify the verb and are essential for constructing different verb tenses and moods.
Step-by-step explanation:
The last two letters of the infinitive form of a verb in English are 'to'. This applies to all English verbs in their infinitive form, which is the base form commonly preceded by the word 'to', such as 'to play', 'to eat', and 'to be'. When dealing with the construction of verbs within sentences, it is important to understand the role of auxiliaries.
Auxiliaries like 'could', 'must', and 'ought to' help form different tenses and moods of verbs and are considered an integral part of the verb phrase, rather than separate adverbs.
An example provided where the infinitive is used with an auxiliary is 'I could finish my essay by eight o'clock tonight'. Here, 'could' is an auxiliary that is attending the verb 'finish', and together they make up the verb phrase.