1. Modal verbs express modality, ability, possibility, need or other condition
2. Modal verbs are auxiliary verbs that can not function as a main verb, unlike the auxiliary verbs "be", "do" and "have" that can function as a main verb.
3. Applications
3.1. "Can" indicates ability or possibility. In these cases it can be translated as "power" in Spanish. Examples: I can speak five languages.
3.1.1. "Could" indicates possibility or ability in the past. Examples: Joe could speak Spanish when he was young You can also use "could" for future possibilities. Examples: You could pass the test if you studied
3.1.1.1. "May" is used to indicate possibilities in the future. Examples: I would bring an umbrella, it may rain later. It can also be used to give permissions or instructions. Examples: You may leave if you like.
3.2. Se usa “might” para indicar posibilidades en el presente o el futuro. En estos casos, es un sinónimo de “may”. Ejemplos: I would bring an umbrella, it might rain later
3.2.1. "will" is used to form the future tense. Also the use of "will" means will or determination. Examples: I will help you "Will" is used in interrogative phrases to ask for information, a favor or about options. Examples: Will you help me move?
3.2.1.1. "Shall" is used as "will" to form the future tense. Examples: Chris shall be happy to see you. You can also use "shall" for offers and suggestions or to ask about options or preferences. Examples: Shall we meet at 10pm
3.3. "Should" indicates an obligation or recommendation. Reflects an opinion about what is right. Examples: I should call my parents more often "Should" is used in interrogative phrases to ask if there is an obligation or to ask for a recommendation. Examples: Should we leave a tip?
3.3.1. "Ought to" is a synonym for "should" although it is less common. Examples: She ought to quit smoking.
3.3.1.1. "Must" indicates an obligation, prohibition or necessity. Examples: You must read this book, it's fantastic You can also use "must" to indicate probability or assume something. Examples: John's not here. I must be sick because I never misses class.