When exploring your options at a car dealership, you might want to ask about "another model," "a similar vehicle," or "such a car." In English, we naturally use the indefinite articles "a" or "an" in all of these phrases. In Portuguese, however, you must omit the indefinite article (um, uma, uns, umas) when using four specific words: outro, semelhante, tal, and certo.
Outro (Another)
Because the English word "another" literally contains the word "an", it is very tempting to translate it as um outro. However, in standard European Portuguese, outro (and its feminine form outra) stands completely on its own. You must drop the um or uma.
Semelhante (A similar) and Tal (Such a)
The same omission rule applies when using semelhante (similar) and tal (such). Where English says "a similar vehicle" or "such a suspension", Portuguese simply says "similar vehicle" (viatura semelhante) and "such suspension" (tal suspensão).
Certo (A certain)
Finally, when you use certo or certa before a noun to mean "a certain" (referring to a specific, though often unnamed, thing), you also leave out the indefinite article.