Learn Portuguese (Portugal): Eu trabalho até às seis.

No, trabalho is in the present tense (I work). You will often see present tense sentences mixed in for review! To change this exact sentence to the past tense (I worked) which you are learning now, you drop the -ar from the dictionary form trabalhar and add the past tense ending -ei, giving you: Eu trabalhei até às seis.

Why is there an accent pointing to the left on às?

The word às is a contraction. To say "at the" before a time, you combine the preposition a (at) with the feminine plural article as (the). This is because the hidden word horas (hours) is feminine and plural. When a and as merge into às, we use that grave accent mark to show the contraction happened.

Why doesn't the sentence say até às seis horas?

Just like in English where we say "until six" instead of "until six o'clock", in Portuguese you can drop the word horas when it is completely obvious from the context that you are talking about the time.

Do I always use até às when saying "until" a certain time?