Must, Have (got) to and Their Negative Forms

MUST and HAVE TO both express necessity. In everyday statements of necessity, have to is used more commonly than must. Must is usually stronger than have to and can indicate urgency.

When used in the negative ‘must‘ and ‘have to‘ have different meanings.

MUST NOT = prohibition (do not do this!)

• You must not tell anyone my secret. Promise?

[Do not tell anyone my secret. I forbid it. Telling anyone my secret is prohibited.]

DO NOT HAVE TO = lack of necessity

 • I can hear you clearly. You don’t have to shout.

[It is not necessary for you to shout because I can hear you clearly.]

Lack of necessity may also be expressed by NEED NOT + verb1.

• You need not shout at me.

• You needn’t worry.

Common Mistakes & Solutions in the Use of USED TO and WOULD

MISTAKE : In the past, I used to call her on her birthday.
SOLUTION: 
In the past, I would call her on her birthday. 
— Use WOULD for a custom. Use USED TO for a discontinued habit.

MISTAKE : Last year, I used to study very hard. (Including an adverb is awkward)
SOLUTIONS:
I used to study very hard.
— Remove the adverb.
Last year, I studied very hard.
— Change the verb to past tense when using an adverb for time.
When I was in college, I used to study very hard.
— Use a time-relative clause to refer to an earlier stage of life.

MISTAKE: I used to live in Missouri for five years. (Including an adverb is awkward)
SOLUTIONS:
I lived in Missouri for five years.
— Use past tense with a quantity of time.
I used to live in Missouri.
— USED TO is not used with a quantity of time.

MISTAKE: I used to go to the gym several times. (Including an adverb is awkward)
SOLUTIONS:
I went to the gym several times.
— Use past tense.
I often used to go to the gym.
— Use an adverb of frequency instead.