Modal verbs are auxiliary verbs that cannot function as a main verb, unlike auxiliary verbs "be", "do" and "have" who can function as a main verb.
Modal verbs express modality. Theycan express ability, possibility, necessity or other condition. Auxiliary verbs are the future and the conditional. English tenses are not enough to express ideas of chance or uncertainty. No one of theconditional tense in English. In turn, auxiliary verbs are used for conditional. Nor is there as the subjunctive tense in Spanish. Therefore it is very important to understand the uses of modalverbs.
As complementary verbs are modal verbs do not work without other verb. This other verb always goes after the modal verb in the base form (infinitive without "to"). No modal verbs are conjugated andhave no time.
"Can" means skill or ability. In these cases it may be translated as "power" in Spanish.
* We can work late tonight if you need us.
In interrogative sentences, the useof "can" may request permission or to inquire about possibilities.
* Can I have a glass of water?
"Could" indicates possibility or ability in the past.
* I couldn't sleep last night.
*It could have been worse.
As "can", in interrogative sentences "could" may request permission or to inquire about the possibilities, but is more formal.
* Could you pass the salt please?
You canalso use "would" for possibilities in the future.
* I think it could rain later.
As "could", using "may" to indicate future possibilities.
* It may be better to finish this now, ratherthan wait until tomorrow.
It can also be used to give permission or instructions.
* You may leave if you like.
Yes or no questions, the use of "may" is more polite than "can".
* May I have a glassof water?
Use "might" to indicate possibilities in the present or the future. In these cases, it is synonymous with "may".
* I would bring an umbrella, it might rain later.