Hello Kem, Can and could are both modal verbs. They are often used in conversation. Can and could are used to show permission, possibility, or ability. Can is more commonly used than could. Could also expresses tentativeness, uncertainty, or politeness in both speech and writing, and is used to convey a high degree of uncertainty in academic writing (Biber, et al., 2002). For a full explanation with examples, click this link: https://drive.google.com/file/d/0B255z0B0MVkUS3FTMG9sVWNCNkk/view?usp=sharing

I know the basics like could is the past tense of can, and the latter is used for present tense. However, when it comes to application, I'm having doubts if I use them properly. Anyway, thank you for the answer. :-)

You've to identified the major ambiguities... "Both can and could (and other modals, especially may and might) are used to express various kinds of possibility, ability, permission and potential." "Could, of course, functions as the past tense of can, and like other past forms, it....can also indicate things like tentativeness, indirectness, deference and a wish not to impose." ...but your response should be split into four fundamental use cases: In the present tense, could expresses possibility (in a particular case)--but so does can (in an ongoing- or general situation): • "I could drink all night and be fully functional in the morning," would refer (in present tense) to tonight; • "I can drink all night and be fully functional in the morning" would refer to any night. Further examples might include: • "We could be looking at a costly citation here," versus "Speeders can expect heavy fines in Texas" As the conditional form of can, could is also commonly used in conditional sentences: • would + can = could Examples might include: • "If we had some eggs I could fix you an omelette;" • "I could wash the car if you'd paid the water bill." As non-deontic indicators of volitivity, can and could introduce increasing degrees of implied uncertainty when used to initiate a request: • "Can you open the window?" and • "Could you open the window?" Each means "Please open the window," with could implying the greater reluctance to impose.