That is correct, THE damsel.
The word almah has a ה before it, what makes it THE almah.
But in a dictionary you won't find "THE almah", only "almah".
By the way, "THE" almah indicates that both Isaiah and Achaz knew her, and that they were not speaking about a woman who would be born 700 years later.
Your understanding is way to simplistic. Your understanding should be that young women in those days COULD be virgins.
Also in those days young woman got seduced,
I thought it was that she was to become pregnant. So, she hadn't been pregnant before?!raped, and, in those days, girls were married of right before or at puberty. So this almah most likely was married,
I think there is another word that starts with a 'B'?this because of the fact that she was pregnant.