Learn Portuguese (Portugal): O peixe ainda está vivo.

In Portuguese, being alive (vivo) or dead (morto) is considered a state or condition, so it takes the verb estar, not ser. It is one of the classic exceptions that English speakers have to memorize, since being alive feels like a permanent characteristic to us!

I have seen ainda used to mean "yet". How do I know it means "still" here?

The meaning of ainda usually depends on whether the sentence is positive or negative. In an affirmative sentence like this one, it means "still" (ainda está vivo = is still alive). If the sentence were negative, it would translate as "yet" (for example, ainda não está pronto = it is not ready yet).

Peixe ends in an "-e". How do we know it is masculine to match with the masculine adjective vivo?

Nouns ending in "-e" do not have a strict rule for gender, so you have to learn them with their article. Here, the article is o, so o peixe is masculine. Because of this, the adjective "alive" must take the masculine singular form, vivo, instead of the feminine viva.

How is the "x" pronounced in peixe?

In peixe, the "x" makes a "sh" sound, like in the English word "shoe". The letter "x" can make several different sounds in Portuguese, but when it comes after a double vowel sound (like "ei"), it almost always makes this "sh" sound.