MUST, HAVE, SHOULD

Should
Se usa "should" para dar o pedir consejo o para dar opiniones en el presente:

• "I have a terrible stomachache." "You should go to the doctor's."
• "I haven’t heard from my father." "You should call him."
• "She's not happy with the salary offered." "She shouldn't accept the job."

Forma

Se usa "should" / "shouldn't" + el infinitivo del verbo principal (sin "to") 

Ejemplo 

I  You  He, She, It  We  They	should	do	more exercise.
		get	a new car.
	shouldn't  (should not)	smoke.
		spend	so much money.

Should	I  you  he / she / it  we  they	get	more qualifications?
		wear	something different?

Have to

Se usa "have to" para mostrar que una persona está obligada a hacer algo, por lo general debido a una fuerza externa, "have to" también se puede usar para expresar opinión:

• You have to show your passport at passport control. 
  (It's the law = Es la ley)
• Jenny has to do homework every evening. 
  (Her parents told her to do her homework = Sus padres le dijeron que hiciera sus deberes)
• Tom had to work late last night. 
  (He hadn't finished his work = no había terminado su trabajo)
• You have to tell him! 
  (Esta es mi opinión (firme))
• You don’t have to eat that if you don’t like it. 
  (I am not obliging you to eat it = No te estoy obligando a que lo comas)

La forma negativa, "don't" / "doesn't" / "didn't have to", significa que uno no está obligado a hacer algo.

Forma

El presente simple de "have" + "to" + infinitivo (sin "to") 
or 
The past simple of 'have' + to + infinitive (without to) 

Ejemplo 

I  You  We  They	have to	leave the house early to catch the bus.  care for the children.
He / She / It	has to
I  You  We  They	don't have to	leave the house early to catch the bus.  care for the children.
He / She / It	doesn't have to

Do	I  you  we  they	have to	work on Saturday?
Does	he / she / it		attend the conference?

Must

"Must" también se usa para hablar de obligación y se usa en reglas e instrucciones escritas:

• You must submit your proposal by noon on 12th July.
• You must not use a calculator during the exam.

"Must" también se usa para hacer una recomendación firme:

• You must tell him before it's too late.
• You mustn't be late for work on your first day.

La forma negativa, "mustn't" ("must not"), significa que usted está obligado a hacer algo o se le recomienda no hacer algo.

Forma

"must" o "mustn't" ("must not") + el infinitivo del verbo (sin "to") 

Ejemplo 

I  You  He / She / It  We  They	must  mustn't	report	the theft to the police immediately.
		forget	to transfer the money.

Must

I  you  he / she / it  we  they

go?

Si bien las preguntas con "must" son gramaticalmente correctas, hoy en día suele ser más habitual usar "have to" en las preguntas. 

"Must" denotando obligación no se suele usar en el pasado:

• We had to show our passports at the border.
• We weren’t allowed to use calculators in the exam. 
  Or, we couldn’t use calculators in the exam.

REALIZA LOS SIGUIENTES LINKS:

1. https://www.aulafacil.com/cursos/ingles/repaso-de-gramatica/ejercicios-should-must-have-to-l22661

2. https://elt.oup.com/student/englishfile/intermediate/a_grammar/file03/nef_int_grammar03_a01?cc=us&selLanguage=en