Sometimes students are taught in school that could is a more formal way to write can. It is not.
In English, can and could are used - most of the time - to show probability:
In English if something happens the same way, all the time, predictably, without variation, then it's 100% probable, and so can is more apporpriate than could. In this case, use present simple tense without can and without could:
Only use can or could when there is an element of uncertainty, and then choose either can or could depending on the degree of certainty:
(Gravity certainly a good method)
(Gravity not necessarily the best method)
Notice that could implies a more negative evaluation than can. Again, Indonesians should think carefully about this distinction as they tend to over-use could, having been taught in school that could is more formal than can. Well, yes it is, but only in offers and requests: