Learn Portuguese (Portugal): Não há nenhuma pista no armário.

In Portuguese, when a negative word like nenhuma (no/not any) comes after the verb, you have to use a double negative. You do this by putting não before the verb. Literally, it translates to "Not there is no clue".

Why does nenhum end in an "a" here (nenhuma)?

Nenhuma is an indefinite determiner, which means it has to match the gender and number of the noun it describes. Because pista (clue) is a feminine singular noun, the base word nenhum changes to its feminine singular form, nenhuma.

Does há mean "there is" or "there are"?

It means both! Há comes from the verb haver. It is used to say that something exists. It only has this one form in the present tense, so you use há whether you are talking about one single thing or multiple things.

I see no armário. Why isn't it em o armário?

In Portuguese, the preposition em (in/on/at) always combines with the definite article (the word for "the") that follows it. Since armário (closet) is masculine, em merges with the masculine "the" (o) to form the single word no.