Hey, these are not prepositions. Actually, they are "combos" of a preposition (A, DE, EM, POR) + some article or demonstrative. Some prepositions can be attached to an article or a personal pronoun or a demonstrative, if they start with a vowel. The top "combo makers" prepositions are "de" and "em". "DE" makes "D" + something Articles: "D" + O, A, OS, AS = DO, DA, DOS, DAS Personal Pronouns: "D" + ELE, ELA, ELES, ELAS = DELE, DELA, DELES, DELAS Demonstratives: "D" + ESTE, ESTA, ESTES, ESTAS, ESSE, AQUELE, ISSO, AQUILO = DESTE, DESTA, DESTES, DESTAS, DESSE, DAQUELE, DISSO, DAQUILO. "EM" makes "N" + something NO, NA, NESTE, NESTA, NESSE, NAQUELE, NISSO, NAQUILO... "A" attaches to articles and demonstratives started with "a" A + (O, A, OS, AS) = AO, À (this is a + a), AOS, ÀS, ÀQUELE (this is A + AQUELE), ÀQUELA, ÀQUELES "POR" attaches to articles only. In in this case it changes into "PEL" PELO (POR + O), PELA, PELOS, PELAS You may see also preposition "PARA" making such combos with articles some demonstratives ("PR" + something), but this is considered colloquial only. Not fully accepted as correct in written texts. PRO, PRA, PROS, PRAS, PRAQUELE... Some examples: I got rid OF THIS problem = Me livrei DESSE problema (DE + ESSE) A casa DELE (DE + ELE) = HIS house (lit. OF HIM) Saia DESSA sala (DE + ESSA) = get out FROM THIS room. Morei NAQUELE apartamento por 3 anos (EM + AQUELE) = I lived IN THAT apartment for 3 yrs. A história foi contada PELAS crianças (POR + AS) = it was told BY THE children. In Portuguese we use articles before names of countries and days of week. We say: o Brasil, os Estados Unidos, a França, a Espanha, o Domingo, a Segunda, a Terça. ("Portugal" is not used with an article) Thus: NO Brasil (= literally IN THE Brasil), NOS Estados Unidos, NA França, EM Portugal. NA segunda = on monday, NO domingo = ON Sunday (lit. ON THE Sunday). Sorry for the English, hope this helps.