How to use 'MUST' in sentences as an Auxiliary Verb?
In the series of learning medical transcription course, we dealt about 'WOULD' usage in the prior lesson. Now we will study about the use of MUST as an auxiliary verb in English grammar.  Uses of  ‘must’: 1. Must is used as an auxiliary to show duty or 
obligation (Here ‘must’ is stronger than ‘should’) 2. Must is used in future in 
the past tense. Let us come to know certain rules which is used in the tense of 
auxiliary form of the verb. A. In the simple present tense form the auxiliary 
‘must’ is used in the present form of the verb. B. In the present continuous 
tense form the auxiliary ‘mustbe’ is used in the present form ending with –ING. 
C. In the future in the past tense the auxiliary ‘must have’ is used in the past 
participle form. D. In the future in the past continuous tense ‘must have been’ 
is used in the present form ending in –ing.
Whenever the structure of the 
sentence expresses duty or obligation, the structures is shown as below 
examples: 1. We must obey the office rules and regulations faithfully. (This is 
in the simple present tense) 2. We must not work against the interests of 
others. (This is in simple present tense) 3. Must we visit all the schools in 
this town everyday? (This is in simple present tense) 4. We must not be waiting 
for too long. (This is in the present continuous tense) 5. You must speak to the 
prinicipal today itself, mustn’t you? (This is in simple present 
tense).
Note: The short form of ‘must not’ is mustn’t. Here the silent letter is first t, so we pronounce the word as musn’t
Note: The short form of ‘must not’ is mustn’t. Here the silent letter is first t, so we pronounce the word as musn’t
Whenever the structure of the 
sentence is in the form of ‘future in the past’ then the examples are shown 
below: 1. Elizabeth must have set a record by now in modeling.
2. All teachers mustn’t have gone for meeting today. Some must 
have stayed behind. 
3. The doctor have been examining 
the patient when you called at my clinic. (This is in the future in the past 
continuous tense) 4. All the members in our family must have returned from 
vacation by today.
 
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