Questions & Answers about Debes tomar dos pastillas al día.
Why is "tomar" used for "to take" here instead of "llevar"?
Could I say "Tienes que tomar" instead of "Debes tomar"?
Why does "a day" translate to "al día" instead of "un día" or "por día"?
When expressing frequency like "per day" or "a week", Spanish uses the preposition a followed by the definite article (el, la, etc.). Because día is masculine, a and el combine into the contraction al. This is why "a day" becomes al día.
Is "día" masculine or feminine since it ends in an "a"?
Día is a masculine word, which is why it pairs with masculine articles like el (creating the contraction al). It is a very common exception to the general rule that words ending in "a" are feminine.