Learn Portuguese (Portugal): Não bebo a mistura, pois sabe a hortelã. - I do not drink the mixture, because it tastes like mint.

In Portuguese, the verb saber has two distinct meanings. While it normally means "to know", adding the preposition a changes its valency. The expression saber a means "to taste of" or "to taste like". Therefore, sabe a hortelã means "it tastes like mint".

The word for mint is feminine (a hortelã). Why didn't the preposition a combine with the definite article to form à?

When describing a general flavor or ingredient, we omit the definite article. We just use the preposition a on its own, so it stays sabe a hortelã. You would only use the contraction à (sabe à hortelã) if you were talking about a specific, physical piece of mint, like saying "it tastes like the mint".

I usually use porque to say "because". Is pois used exactly the same way here?

In the middle of a sentence, pois works exactly like porque to introduce a reason or justification, though it can sound slightly more formal or written. However, they are not always perfectly interchangeable: unlike porque, you cannot use pois at the very beginning of a sentence to answer a question.