Learn Portuguese (Portugal): A minha mãe gosta do chá preto.

The verb gostar (to like) always links to what you like using the preposition de. Because we are talking about a specific tea, "the" black tea, we need the masculine word for "the" (o). In Portuguese, de and o squeeze together to make the single word do.

Why is there an A at the very beginning of the sentence?

In European Portuguese, you usually put a definite article (the word for "the") before possessive words like minha (my). Because mãe is a feminine noun, we use the feminine A at the start. Literally, the sentence starts with "The my mother".

Why are the words for "black tea" swapped around as chá preto?

In Portuguese, adjectives (words that describe things, like colors) almost always come after the noun they are describing. So instead of saying "black tea" like in English, you say "tea black" (chá preto).

If the verb is gostar, why is the form gosta used here?

Verbs change their endings based on who is doing the action. gosto is the form for "I", meaning "I like". Because the subject here is A minha mãe (she), we use the "she" form of the verb, which is gosta.