The letter and word a in both English and Spanish

In English, the letter as a word and as an affix has several meaning.  In Spanish, when being translated into English, a, especially as a preposition does not have just one translation.  Wylcomenetwork.com has bilingual istudycards available to help the Spanish-speaker to understand how a is used in English and the English-speaker to get a taste of many ways, the preposition a is used in Spanish.  If one can say such a thing, in Spanish, of all the preposition of the language, the a is the most important.