Learn Portuguese (Portugal): Ela nunca recebeu essa carta.

Recebeu is the simple past tense of the regular verb receber (to receive). To make the ela (she) form for an -er verb in the past tense, you drop the -er ending and add -eu.

Why isn't there a não in this sentence?

Because nunca (never) is already a negative word. In Portuguese, when nunca comes right before the verb, it makes the sentence negative all by itself, so you do not need to add não.

Why does the sentence use essa instead of aquela for 'that'?

Essa refers to something close to the person you are talking to, or a topic that was just brought up in conversation. You would use aquela if the letter were far away from both of you.

How is the double 's' in essa pronounced?

The double 's' in essa makes a soft 's' sound, like in the English word 'hiss'. If it only had a single 's' between two vowels, it would sound like a 'z'.