What Can the English Word "Can" Mean and How to Get it Right in Spanish
Can, Could and More
I'll start with a funny story.
Once, after teaching a review lesson about helping verbs plus infinitives, I assigned only one sentence for the following day (they were struggling). The sentence was I can go. They were to translate it into Spanish. At least half the class (of 25) got it right: Puedo ir. I also accepted Yo puedo ir. However, the range and types of wrong answers revealed a number of conceptual deficits, or perhaps attention problems. Imagine my surprise when I saw Yo lata ir and Yo lata voy or lato ir... and so forth. The word lata means a can, as in a tin can.
There is a moral to this story, for teachers and students. At the most basic level, that of knowing parts of speech and how to use a dictionary correctly, several freshmen had been sorely left behind. Some of the other errors also revealed no knowledge of conjugation or no recollection that only the helping verb is to be conjugated. One decided that lata (the noun) could be made into a verb (hypothetically latar!)... So, be careful when classifying the function of a word. Dictionaries always will indicate the part of speech, so be familiar with their abbreviations.
As for how to deal with can -- a helping or modal verb which in English refers to an ability or capacity to do something, the verb poder is usually the solution, but remember that in the present tense (indicative or subjunctive) it means, simply can or, as in asking permission, may.
Mi amigo puede acompañarnos esta tarde (My friend can go with us this afternoon).
No puedo ver bien desde aquí (I can't see well from here).
Mamá, ¿podemos ir a la playa mañana? (Mom, may we go to the beach tomorrow?)
In the conditional, it means could. It is the form to be used when expressing the consequence of a hypothetical statement (the imperfect subjunctive is used in the if-clause).
Podría hacerlo si tuviera las herramientas (I could do it if I had the tools).
When used in questions, the use of the conditional instead of the present indicative makes the request more polite.
¿Podrías ayudarme con la tarea? (Could you [please] help me with my homework?)
Using the imperfect subjunctive makes a question even more polite. This feature of the language is shared by poder, querer and deber and is known as degrees of politeness.
¿Pudieran invitar a mi amigo Jorge? (Might you all be so kind as to invite my friend George?)
Finally, the idea of can is often expressed with another verb: saber. This is because when saber is followed by an infinitive, it means to know how to do something.
Mis amigos saben tocar la guitarra (My friends can/know how to play the guitar).
¿Sabes nadar? (Can you/Do you know how to swim?)