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MODAL VERBS

The modal verbs are a class of auxiliary verbs. Modal auxiliary verbs are verbs which 'help' other verbs. Words are used with other verbs to express different functions such as obligation, ability, advice, opinion, prohibition ... and more. The following are modal auxiliary verbs:

can - could - may - should - would - must - ought to

In a Affirmation, the word order is Subject + Modal verb + Main verb

Example:

• They can come (Ellos pueden venir)

In the Questions, the word order is Modal verb + Subject + Main Verb

Yes o no questions:

• Can they come? (¿Pueden venir?)

Wh- questions:

• When can they come? (¿Cuándo pueden venir?)

CAN
The modal can, in spanish "poder", indicate a possibility or capacity.

• Tom can help you (Tom puede ayudarte)
• Eating out can be costly (Comer fuera puede ser costoso)

in questions, can be used to request permission to do something or to ask about a possibility.

• Can I help you? (¿Puedo ayudarte?)
• Can Mike have dinner with us? (¿Mike puede cenar con nosotros?)

COULD
The modal could indicate a possibility or capacity in the past.

• I could have told you that (Podría haberte dicho eso)
• I could run very fast (yo podía correr muy rápido)

Could is used to speculate about future possibilities.

• It could rain tonight (Puede llover esta noche)
• This could be dangerous (Eso puede ser peligroso)

In the questions yes or not, could speculate on some possibilities in the present or the future.

• Could this be a mistake? (¿Podría ser esto un error?)

Could indicate an option.

• We could go see movie (Podríamos ver una pelicula)

MAY
The modal may indicate an action that can happen in the future.

• This may be a bad idea (Quizás esto sea una mala idea)
  

MUST
The modal must indicates obligation.

• Tom must see a doctor immediately (Tom debe ver un médico inmediatamente)

Must can indicate probability or assumptions.

• You must be angry (Estará enojado)

In information questions, the word must indicates a requirement and may be Synonymous with should. In American English is much more common should in this context.

• When should / must we be there? (¿Cuándo debemos estar allá?)

SHOULD and OUGHT (TO)
The modals should and ought to indicate an obligation, and both are synonymous.

• You should / ought to call your mother (Debes llamar a tu mamá)

In questions should is used to determine whether an obligation exists. In American English ought never used in questions

• Should he call her? (¿Debe llamarle a ella?)

WOULD
The modal would followed by the word like is a polite way of indicating a preference.

• I would like white wine with my fish (Me gustaría el vino blanco con mi pescado)

In questions, would like is a polite way to request a decision.

• Where would you like to eat dinner? (¿Dónde le gustaría cenar?)